UC Berkeley

Cell-cell fusion is indispensable for numerous stages of development, from fertilization to tissue development and maintenance. It is typically driven by fusogenic membrane proteins that have tall (>10 nm) ectodomains that bridge the distance between plasma membranes. These tall fusogens often resemble those involved in enveloped viral entry and undergo conformational changes that pull the two membranes into close contact and drive fusion. In contrast, some cell-cell fusogens, such as the fusion-associated small transmembrane (FAST) proteins from reovirus, have ectodomains that are short (<2 nm), smaller than the repulsive hydration barrier that prevents membranes from coming into close contact.

In this dissertation, I investigated the mechanism of FAST protein-mediated cell-cell fusion. In the first part, I discovered that p14, the FAST protein from reptilian reovirus, hijacks the actin cytoskeleton by binding to the adaptor Grb2 through a phosphorylation-dependent motif in its cytoplasmic tail to drive cell-cell fusion. In the second part, I examined a related FAST protein that arose from a distinct gain-of-function evolutionary event more than 500 million years ago might and found that it also usurped the cellular actin machinery to drive cell-cell fusion. Findings from both studies suggest that these short cell-cell fusogens use force generated from localized actin assembly to overcome the intermembrane gap and bring the two membranes in close contact for fusion. This work establishes how short, non-canonical fusogens can use an alternative, non-conformational change-based mechanism to drive fusion of crowded plasma membranes, providing a mechanistic model for other short, physiological cell-cell fusogens like myomixer and myomaker.