A myristoyl switch at the plasma membrane triggers cleavage and oligomerization of Mason-Pfizer monkey virus matrix protein
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eLife assessment
This valuable study advances our understanding of how the viral protease in a D-type retrovirus is activated and in particular how the exposure of the myristoyl group is required for processing of the Gag matrix precursor. The supporting evidence is convincing, but the work would benefit from additional data in support of the claims. This manuscript is of interest to retrovirologists and structural biologists.
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Abstract
For most retroviruses, including HIV, association with the plasma membrane (PM) promotes the assembly of immature particles, which occurs simultaneously with budding and maturation. In these viruses, maturation is initiated by oligomerization of polyprotein precursors. In contrast, several retroviruses, such as Mason-Pfizer monkey virus (M-PMV), assemble in the cytoplasm into immature particles that are transported across the PM. Therefore, protease activation and specific cleavage must not occur until the pre-assembled particle interacts with the PM. This interaction is triggered by a bipartite signal consisting of a cluster of basic residues in the matrix (MA) domain of Gag polyprotein and a myristoyl moiety N-terminally attached to MA. Here, we provide evidence that myristoyl exposure from the MA core and its insertion into the PM occurs in M-PMV. By a combination of experimental methods, we show that this results in a structural change at the C-terminus of MA allowing efficient cleavage of MA from the downstream region of Gag. This suggests that, in addition to the known effect of the myristoyl switch of HIV-1 MA on the multimerization state of Gag and particle assembly, the myristoyl switch may have a regulatory role in initiating sequential cleavage of M-PMV Gag in immature particles.
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eLife assessment
This valuable study advances our understanding of how the viral protease in a D-type retrovirus is activated and in particular how the exposure of the myristoyl group is required for processing of the Gag matrix precursor. The supporting evidence is convincing, but the work would benefit from additional data in support of the claims. This manuscript is of interest to retrovirologists and structural biologists.
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Reviewer #1 (Public Review):
The hypothesis that the MA myristyl switch is a trigger for M-PMV maturation is derived from previously published findings, and is well supported by the data presented in this manuscript. The results suggest a new function for the myristyl switch, one that could perhaps be relevant for other proteins. Below are some suggestions for improving the MS.
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Reviewer #2 (Public Review):
This manuscript presents measurements of proteolytic digestion and structural studies using both hydrogen-deuterium exchange and NMR. The data test the idea that membrane association leads to a rearrangement of the MA domain of the MPMV Gag protein, as the myristate chain at the N-terminus of the protein is "switched" from a hydrophobic pocket within the protein into lipid layers, finally rendering the protein efficiently digestible by the viral protease. In my opinion, the data are highly convincing, and the underlying hypothesis is a useful contribution to the field, providing for this retrovirus a solution to the long-standing problem of how proteolytic maturation is activated.
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Reviewer #3 (Public Review):
D-type retroviruses, which include M-PMV assemble in the cytosol, however, do not efficiently start their maturation before membrane binding. There is very little known about the structural changes leading to maturation of D-type retroviruses and this manuscript presents compelling structural changes of the M-PMV matrix domain in mutations abrogating the myristol exposure or mutation which reasonably argue that myristol group is exposed (The relationship between these mutants and myristol exposure is argued based on structure of the matrix and liposome binding, however is not directly shown in structure). Assuming that the authors are correct about their mutations affect on myristol exposure, they have measured very interesting M-PMV matrix domain conformational changes which exposes the MAPP site to the …
Reviewer #3 (Public Review):
D-type retroviruses, which include M-PMV assemble in the cytosol, however, do not efficiently start their maturation before membrane binding. There is very little known about the structural changes leading to maturation of D-type retroviruses and this manuscript presents compelling structural changes of the M-PMV matrix domain in mutations abrogating the myristol exposure or mutation which reasonably argue that myristol group is exposed (The relationship between these mutants and myristol exposure is argued based on structure of the matrix and liposome binding, however is not directly shown in structure). Assuming that the authors are correct about their mutations affect on myristol exposure, they have measured very interesting M-PMV matrix domain conformational changes which exposes the MAPP site to the protease.
Oligomerization of the matrix is probed by formation of disulfide bridges in a matrix mutant on liposomes with engineered cystine where authors suspect monomers of the matrix would be touching each other. The oligomerization data is very weak, does not directly support trimer formation and since 2D diffusion on liposomes would increase matrix-matrix interactions, can be non-specific, a point supported by presence of a stronger dimer band than trimer and tetramer. The main issue with the manuscript is that the authors do not show any evidence that the proposed mechanism actually works in the context of full M-PMV assembled particles.
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