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Mad1
Crystal structure, tetramer of Mad1-Mad2 complex, yellow and red=Mad1 monomers, palegreen= Mad2 monomers
Identifiers
SymbolMAD1
NCBI gene8379
PDB1GO4
UniProtQ9Y6D9
Search for
StructuresSwiss-model
DomainsInterPro


Mad1 is a non-essential protein (THIS IS TRUE IN YEAST BUT NOT IN THE MOUSE FOR EXAMPLE...) which is part of the spindle assembly checkpoint (SAC)(A CHECKPOINT IS A PROCESS, A PROTEIN IS MOLECULE, THEREFORE IT CAN FUNCTION IN A PROCESS BUT IS NOT PART OF A PROCESS, BECAUSE IT IS NOT A PROCESS ITSELF..., PLEASE REPHRASE) and prevents cells from starting anaphase until the spindle is built up (THIS IS WHAT THE SAC DOES, AND NOT WHAT MAD1 ITSELF-ALONE- DOES, YOUR SENTENCE IS CONFUSING). Mad1 recruits Mad2 to unattached kinetochores (DEFINE RAPIDLY MAD2 THEN, FOR EXAMPLE SAY, THE ANAPHASE INHIBITOR MAD2) and is essential for Mad2-Cdc20 complex formation (DEFINE CDC20 NOW) in vivo but not in vitro. (IN SUMMARY REWRITE THIS PARAGRAPH SUCH THAT IT CAN BE SELF-CONTAINED AND REFERS TO ALREADY EXISTING PAGES IF THERE ARE ANY). Mad1 is a competitive inhibitor of the Mad2-Cdc20 complex (THIS PUT ALONE IS A LITTLE MISLEADING SINCE MAD1 IS ALSO AN ACTIVATOR OF MAD2... THIS SENTENCE NEEDS TO BE REPHRASED). [1] The name Mad refers to the observation that mutant cells are mitotic arrest deficient (MAD) during the presence of microtubule depolymerization (PERHAPS YOU COULD START WITH THAT, RIGHT AFTER SAYING THAT MAD1 IS PROTEIN FUNCTIONING IN THE SAC). The phosphorylated form of Mad1, is phosphorylated by Mps1 (MISLEADING: AS YOU WRITE IT IT MAY MEAN THAT AFTER BEING PHOSPHORYLATED BY AN UNNAMED KINASE, MPS1 PHOSPHORYLATES FURTHER PHOSPHO-MAD1) which then leads to inhibition of the anaphase-promoting complex(cyclosome) (APC/C)activity (NOT DIRECTLY...). Homolog’s of Mad1 are conserved in eukaryotes (BETTER SAID, THEN: MAD1 IS CONSERVED IN EUKARYOTES- YOU HAVE NOT MENTIONED ANY ORGANISM AT THIS POINT AND IF YOU WANTED TO SUGGEST THAT MAD1 IS A YEAST PROTEIN 1- IT IS ALSO NAMED MAD1 IN MOST IF NOT ALL OTHER ORGANISMS, 2-YOU HAVE NOT SAID SO FAR THAT MAD1 IS A YEAST PROTEIN).


Introduction

In the early 90s, YEAST genes were identified which mutations resulted in A DEFECT IN MITOTIC ARREST IN RESPONSE TO MICROTUBULE DISASSEMBLY (mitotic arrest deficient GENES-MAD GENES). This cells showed during division no mitotic arrest in the presence of microtubule polymerization inhibitors and were therefore not able to delay cell division[2]. The genes identified included the MAD1, MAD2 and MAD3 genes. They are conserved in all eukaryotes and are involved in a pathway that is active in prometaphase to prevent the premature separation of sister chromatides and constitute the so called spindle assembly checkpoint(SAC). This checkpoint monitors the status of CHROMOSOME ATTACHMENT TO THE MITOTIC SPINDLE the spindle and INHIBIT the metaphase to anaphase transition by preventing the ACTIVATION OF THE anaphase-promoting complex/cyclosome (APC/C), AND THEREBY THE degradation of cell cycle regulators [3]. HERE SAY RAPIDLY (ONE SENTENCE) WHERE IS MAD1 LOCATED IN THIS PATHWAY (AT KINETOCHORES AND QUITE UPSTREAM IN THE SENSING MACHINERY)

Function

Fig. 2. Mad1 function in SAC. Mad1 homodimer in unattached kinetochores is bound to two c-Mad2 and forms a catalytic receptor for cytozolic o-Mad2. Complex Mad1-cMadD2-oMad2 catalyzes conformational change of inactive oMad2 to the active c-Mad2 form. C-Mad2 then binds to Cdc20 and mediates APC/C inhibition and mitotic arrest.

Dividing yeast (ALL EUKARYOTIC CELLS) cells show a mitotic arrest in the presence of microtubule polymerization inhibitors. A spindle assembly checkpoint monitors the status of the spindle and arranges (ARRANGING IS NOT AN APPROPRIATE WORD HERE, SEE ENGLISH DEFINITION OF THAT WORD. A BETTER WORD WOULD BE "CONTROLS" BUT EVEN BETTER WOULD BE TO SAY: "LINKS THE METAPHASE-ANAPHASE TRANSITION TO PROPER BIPOLAR ATTACHMENT OF ALL KINETOCHORES TO THE MITOTIC SPINDLE") metaphase to anaphase transition. This spindle assembly checkpoint inhibits the activity of the anaphase promoting complex by preventing degradation of the downstream effectors leading to ANAPHASE ONSET AND exit from mitosis. Depletion of Mad1 leads to loss of SAC function. Mad1 is necessary for kinetochore localization of important SAC COMPONENT Mad2 (Fig. 2). There is a pool of free cytoplasmic Mad2 where it exists in its inactive open conformation called o-MAD2. When bound to Mad1, Mad2 adopts active conformation called closed (c-Mad2) and forms heterotetramer of two MAD1 and two c-Mad2 units. Heterotetramer of Mad1–c-Mad2 is very stable and works as a catalytic receptor for free cytoplasmic o-Mad2. Free o-Mad2 binds to this receptor and changes its conformation to active closed form. This second c-MAD2 is transferred to Cdc20 with yet unknown mechanism and forms Cdc20–c-Mad2 complex. This complex is essential component of mitotic checkpoint complex MCC. MCC binds and inhibits APC/C and therefore arrests progression through mitosis [4], [3]. WHAT IS MOST IMPORTANT TO MENTION HERE IS THAT MAD1 IS RECRUITED AND ACTIVE MAINLY ON UNATTACHED KINETOCHORES!! THEREFORE, MAD1 LINKS MCC FORMATION TO THE EXISTENCE OF A SINGLE UNATTACHED KINETOCHORE.

Regulation

There are two upstream checkpoint kinases that are implicated in regulating MAD1 function through phosphorylation [5]. Mps1 phosphorylate Mad1 both in vitro and in vivo and is thought to regulate Mad1 and Mad2 localization to kinetochores and their interaction dynamics. BUB1 is the other kinase that recruits Mad1 to kinetochore and activates it if kinetochore is unattached [3]. If kinetochore is attached to spindle, SAC inhibitor p31comet inhibits Mad1 mediated conformational rearrangement of Mad2 and prevents Mad2 from binding to Cdc20 [3].

Structural Features and Mechanism

Fig. 2 Crystal structure, dimer of Mad1-Mad2 complex, yellow and red=Mad1 monomers, palegreen= Mad2 monomers

By biochemical methods Mad1 was predicted to encode a 90kD, 718-residue, [6] coiled-coil protein with a characteristic rod shape [2] in 1995. Chrystal structures followed soon. Then in 2002 the crystal structure of human Mad1 in complex with human Mad2 forming a tetramer was published (figure 1). Due to experimental limitations the structure only shows Mad1 residues 484 - 584. Elongated Mad1 monomers are tightly held together by a parallel coiled-coil involving the N-terminal alpha helices. The Mad1 chains point away from the coiled-cloil towards their Mad2 ligands forming two sub-complexes with Mad2. The segment between alpha helices 1 and 2 contains the Mad2 binding domain (figure 2). The first part of this binding domain is flexible and adopts different conformations giving rise to an asymmetric complex. In their work, employing thermodynamic studies, Sironi et al. [1] show that Mad1 is engineered (?? BY WHICH ENGINEER???? IS THIS IN SUPPORT OF CREATIONISM OR OF INTELLIGENT DESIGN?...) to slow down the rate of Mad2-Cdc20 complex formation and therefore acts as a competitive inhibitor (THIS IS WHAT IS OBSERVED IN VITRO, BUT THIS MAY NOT BE THE CASE IN VIVO, WHERE IT WOULD NOT MAKE COMPLETE SENSE ALONE...). Furthermore the authors suggest, the Mad1-Mad2 binding sites are buried inside the structure perhaps rendering the binding sites inaccessible for Cdc20 binding. Mad1-Mad2 binding is unusual in that the Mad2 C-terminal folds over Mad1. The authors therefore conclude that an unperturbed Mad1-Mad2 complex will not release Mad2 requiring a novel mechanism of conformational change [1]. YES, WHY IS THIS IMPORTANT?

Cancer

Mismatches in chromosome number (aneuploidies) during meiosis are responsible for human diseases like Down´s syndrome and also emerges frequently in cancer cells. The essential function of SAC gives rise to the hypothesis that mutations of the SAC and especially inactivation of SAC might be a reason for tumorigenesis or at least facilitate tumorigenesis[3]. Furthermore (MORE ACCURATE WOULD BE TO SAY. AGAINST THIS IDEA, it was shown that cancer cells undergo apoptosis when components of the SAC are not present[7]. In this model by contrast SAC inactivation becomes a potential way to kill rapidly dividing cancer cells. Summarizing (THIS WORD IS NOT VERY MEANINGFUL WHEN ALONE: SUMMARIZING WHAT?, SIMPLY DROP IT, YOU DON'T NEED IT), the molecular links between Mad1p, the SAC, apoptosis and cancer are still TO BE uncovered[3].

See also


References

  1. ^ a b c L. Sironi, M. Mapelli, S. Knapp1, A. De Antoni, K. Jeang and A. Musacchio (2002). "Crystal structure of the tetrameric Mad1-Mad2 core complex: implications of a `safety belt'". EMBO. 21: 2496–2506. {{cite journal}}: line feed character in |author= at position 61 (help); line feed character in |title= at position 51 (help)CS1 maint: multiple names: authors list (link) CS1 maint: numeric names: authors list (link)
  2. ^ a b Hardwick KG, Murray AW (1995). "Mad1p, a phosphoprotein component of the spindle assembly checkpoint in budding yeast". J. Cell Biol. 131 (3): 709–20. PMC 2120625. PMID 7593191. {{cite journal}}: Unknown parameter |month= ignored (help) Cite error: The named reference "Hardwick" was defined multiple times with different content (see the help page).
  3. ^ a b c d e f Musacchio A, Salmon ED (2007). "The spindle-assembly checkpoint in space and time". Nat. Rev. Mol. Cell Biol. 8 (5): 379–93. doi:10.1038/nrm2163. PMID 17426725. {{cite journal}}: Unknown parameter |month= ignored (help) Cite error: The named reference "Musaccio" was defined multiple times with different content (see the help page).
  4. ^ Yu H (2006). "Structural activation of Mad2 in the mitotic spindle checkpoint: the two-state Mad2 model versus the Mad2 template model". J Cell Biol. 173 (2): 153–157. PMID 16636141. {{cite journal}}: Unknown parameter |month= ignored (help)
  5. ^ Bharadwaj R, Yu H (2000). "The spindle checkpoint, aneuploidy, and cancer". Oncogene. 23 (11): 2016–27.
  6. ^ R. Chen, A. Shevchenko, M. Mann, and A. Murray (1998). "Spindle Checkpoint Protein Xmad1 Recruits Xmad2 to Unattached Kinetochores". The Journal of Cell Biology. 134: 283–295. {{cite journal}}: line feed character in |title= at position 42 (help)CS1 maint: multiple names: authors list (link)
  7. ^ Kops GJ, Foltz DR, Cleveland DW (2004). "Lethality to human cancer cells through massive chromosome loss by inhibition of the mitotic checkpoint". Proc. Natl. Acad. Sci. U.S.A. 101 (23): 8699–704. doi:10.1073/pnas.0401142101. PMC 423258. PMID 15159543. {{cite journal}}: Unknown parameter |month= ignored (help)CS1 maint: multiple names: authors list (link)

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