The regulation role of host cell calmodulin in late phase of retroviral life cycle

Retroviruses comprise a group of viruses characterized by the use of reverse transcriptase to convert their genome ssRNA to DNA in host cells. The mechanisms that retroviruses use in host cells to proliferate are similar to all retrovirus species, but not identical. Enormous efforts have been devoted to uncovering the mechanisms that the human immunodeficiency virus (HIV) uses in individual parts of its life cycle. Nevertheless, there are still unclear processes in the life cycle of these viruses, for example the exact mechanism of the interaction of viral structural polyprotein Gag with the cytoplasmic membrane or the way of regulation of virus maturation.

To better understand the just-mentioned processes in the life cycle of retroviruses, we focused on the interaction between the matrix protein (MA) of Mason-Pfizer monkey virus (M-PMV) and host cell calmodulin (CAM). Matrix protein is N-terminal domain of structural polyprotein Gag that mediates the binding of Gag to cytoplasmatic membrane prior to budding of newly formed viral particles from host cell and their maturation. The impact of the interaction between MA and human CAM for the binding of retroviral precursor Gag to the cytoplasmic membrane has already been reported for HIV. However, the structure of the MA-CAM complex has not been described yet. In our study, we found out that the presence of CAM increases the interaction of M-PMV myrMAPPHis construct (myristoylated N-terminal part of Gag polyprotein with preserved cleavage site for viral protease) to cytoplasmatic membrane. Our data also shows that the oligomerization of M-PMV myrMAPPHis as well as its tendency to be cleaved by viral protease are more effective in the presence of calmodulin. These findings lead us to hypothesize that CAM interaction with MA stimulate the conformation changes of MA to enhance the aforementioned processes and this way serves as the regulation factor in the retroviral life cycle.

To confirm our hypothesis, we applied protein cross-linking that enabled us to confirm the direct interaction between M-PMV myrMAPPHis and human CAM together with revelation the protein regions which are in the complex in close proximity. Interestingly, the N-terminal and C-terminal parts of MA were identified to be in close proximity to N-terminal region of human CAM. On the N-terminus of MA there is post-translationally bound myristic acid which is supposed to play a role in the interaction with the membrane through the myristic switch mechanism and on C-terminal part the specific cleavage site for viral protease occur.