ESCRT (Endosomal Sorting Complex Required for Transport) machinery drives different cellular processes such as endosomal sorting, organelle biogenesis, vesicular trafficking, maintenance of plasma membrane integrity, membrane fission during cytokinesis and enveloped virus budding. The normal cycle of assembly and disassembly of some ESCRT complexes at the membrane requires the AAA-ATPase vacuolar protein sorting 4 (Vps4p). A number of ESCRT proteins are hijacked by clinically significant enveloped viruses including Ebola, and Human Immunodeficiency Virus (HIV) to enable enveloped virus budding and Vps4p provides energy for the disassembly/recycling of these ESCRT proteins. Several years ago, the failure of the terminal budding process of HIV following Vps4 protein inhibition was published; although at that time a detailed understanding of the molecular players was missing. However, later it was acknowledged that the ESCRT machinery has a role in enveloped virus budding from cells due to its role in the multivesicular body (MVB) sorting pathway. The MVB sorting pathway facilitates several cellular activities in uninfected cells, such as the down-regulation of signaling through cell surface receptors as well as the process of viral budding from infected host cells. In this review, we focus on summarising the functional organisation of ESCRT proteins at the membrane and the role of ESCRT machinery and Vps4p during MVB sorting and enveloped viral budding.

Classic functions of the actin cytoskeleton include control of cell size and shape and the internal organization of cells. These functions are manifest in cellular processes of fundamental importance throughout biology such as the generation of cell polarity, cell migration, cell adhesion, and cell division. However, studies in the unicellular model eukaryote Saccharomyces cerevisiae (Baker's yeast) are giving insights into other functions in which the actin cytoskeleton plays a critical role. These include endocytosis, control of protein translation, and determination of protein 3‐dimensional shape (especially conversion of normal cellular proteins into prions). Here, we present a concise overview of these new “moonlighting” roles for the actin cytoskeleton and how some of these roles might lie at the heart of important molecular switches. This is an exciting time for researchers interested in the actin cytoskeleton. We show here how studies of actin are leading us into many new and exciting realms at the interface of genetics, biochemistry, and cell biology. While many of the pioneering studies have been conducted using yeast, the conservation of the actin cytoskeleton and its component proteins throughout eukaryotes suggests that these new roles for the actin cytoskeleton may not be restricted to yeast cells but rather may reflect new roles for the actin cytoskeleton of all eukaryotes. © 2014 IUBMB Life, 66(8):538–545, 2014

The budding yeast Saccharomyces cerevisiae has an actin cytoskeleton that comprises a set of protein components analogous to those found in the actin cytoskeletons of higher eukaryotes. Furthermore, the actin cytoskeletons of S. cerevisiae and of higher eukaryotes have some similar physiological roles. The genetic tractability of budding yeast and the availability of a stable haploid cell type facilitates the application of molecular genetic approaches to assign functions to the various actin cytoskeleton components. This has provided information that is in general complementary to that provided by studies of the equivalent proteins of higher eukaryotes and hence has enabled a more complete view of the role of these proteins. Several human functional homologues of yeast actin effectors are implicated in diseases. A better understanding of the molecular mechanisms underpinning the functions of these proteins is critical to develop improved therapeutic strategies. In this article we chose as examples four evolutionarily conserved proteins that associate with the actin cytoskeleton: (1) yeast Hof1p/mammalian PSTPIP1, (2) yeast Rvs167p/mammalian BIN1, (3) yeast eEF1A/eEF1A1 and eEF1A2 and (4) yeast Yih1p/mammalian IMPACT. We compare the knowledge on the functions of these actin cytoskeleton-associated proteins that has arisen from studies of their homologues in yeast with information that has been obtained from in vivo studies using live animals or in vitro studies using cultured animal cell lines.

Sorting of membrane proteins into intralumenal endosomal vesicles, multivesicular body (MVB) sorting, is critical for receptor down regulation, antigen presentation and enveloped virus budding. Vps4 is an AAA ATPase that functions in MVB sorting. Although AAA ATPases are oligomeric, mechanisms that govern Vps4 oligomerization and activity remain elusive. Vps4 has an N‐terminal microtubule interacting and trafficking domain required for endosome recruitment, an AAA domain containing the ATPase catalytic site and a β domain, and a C‐terminal α helix positioned close to the catalytic site in the 3D structure. Previous attempts to identify the role of the C‐terminal helix have been unsuccessful. Here, we show that the C‐terminal helix is important for Vps4 assembly and ATPase activity in vitro and function in vivo, but not endosome recruitment or interactions with Vta1 or ESCRT‐III. Unlike the β domain, which is also important for Vps4 assembly, the C‐terminal helix is not required in vivo for Vps4 homotypic interaction or dominant‐negative effects of Vps4–E233Q, carrying a mutation in the ATP hydrolysis site. Vta1 promotes assembly of hybrid complexes comprising Vps4–E233Q and Vps4 lacking an intact C‐terminal helix in vitro. Formation of catalytically active hybrid complexes demonstrates an intersubunit catalytic mechanism for Vps4. One end of the C‐terminal helix lies in close proximity to the second region of homology (SRH), which is important for assembly and intersubunit catalysis in AAA ATPases. We propose that Vps4 SRH function requires an intact C‐terminal helix. Co‐evolution of a distinct Vps4 SRH and C‐terminal helix in meiotic clade AAA ATPases supports this possibility.