Filamin A protein interacts with human immunodeficiency virus type 1 Gag protein and contributes to productive particle assembly. host cells and in cell-cell transmission events. IMPORTANCE Viruses often interact with the cellular cytoskeletal machinery in order to deliver their components to the site of assembly and budding. This study indicates that a key regulator of actin dynamics at the plasma membrane, LIM kinase, is important for the release of viral particles for HIV as well as for particle release by a distantly related retrovirus, Mason-Pfizer monkey virus. Moreover, disruption of LIM kinase greatly diminished the spread of HIV from cell to cell. These findings suggest that LIM kinase and its dynamic modulation of the actin cytoskeleton in the cell may be an important host factor for the production, release, and transmission of retroviruses. INTRODUCTION The cellular actin network is critical for cell morphogenesis, cell migration, distribution of organelles, and other fundamental cellular functions (1, 2). Actin exists in the cell in two different forms: actin monomers (G actin) and actin filaments (F actin). Actin filaments undergo dynamic polymerization and depolymerization that produce organized protrusions such as filopodia, lamellipodia, microvilli, podosomes, and membrane ruffles (3). These structures are modified by a number of actin binding proteins and by members of the Rho protein Coluracetam family of small GTPases. Many pathogens have developed strategies to regulate the actin cytoskeleton in order to harness the mechanical forces generated by polymerizing/depolymerizing actin filaments to exit the infected cell and spread from one cell to another. and induce actin polymerization through specific F-actin polymerizing proteins, leading to the formation of actin comet tails that propel the bacteria through the host cytoplasm and into target cells (4). Vaccinia virus (VV) exits the host cell and Rabbit Polyclonal to MCPH1 also induces formation of dense actin comet tails underneath virions, which drive the virions away from the cell and are important for cell-cell dissemination (5). Measles virions budding off the plasma membrane are often associated with cortical actin filaments. These actin filaments exclusively protrude into virus particles and interact with viral nucleocapsids (6). Moloney murine leukemia virus (MLV) has been shown to transit along filopodial bridges from an infected cell toward noninfected cells (7, 8). There is considerable circumstantial evidence for the involvement of actin in HIV particle assembly or release. Actin and the actin-related proteins ezrin and cofilin are highly represented in HIV-1 virions (9). Cryo-electron tomographic analysis of HIV-1 assembly sites revealed that one-half of the HIV budding sites were present on actin-filled filopodia, where actin filaments were aligned toward the budding sites (10). Studies employing inhibitors of actin polymerization such as cytochalasin D and latrunculin B generally showed only a modest decrease in virus release (11, 12), while cell-cell transmission has been shown to be more significantly disrupted by these inhibitors (12). LIM kinase 1 (LIMK1) is a serine protein kinase involved in the regulation of Coluracetam actin polymerization. Once activated, LIM kinase phosphorylates and inactivates the actin depolymerizing factor cofilin, which results in an increase in filamentous actin (13, 14). Here we identified LIMK1 and its upstream activator ROCK1 as cellular factors regulating HIV-1 and Mason-Pfizer monkey virus (M-PMV) release. Both particle release and cell-cell viral transmission were greatly reduced following LIMK1 depletion. Remarkably, depletion of LIMK1 or its upstream regulator ROCK1 caused an accumulation Coluracetam of mature HIV-1 virions.