The myosin in question, Myo5, is a class I myosin from budding yeast. The Boston group found that Myo5 had unusual dynamics: it was stationary at cortical actin patches, the presumptive endocytic sites in yeast, and departed after only a brief stay. The peak of Myo5 localization came immediately before the actin polymerization factor Arp2 switched from slow to fast movement away from the membrane. This switch was delayed (although the fast phase, once started, was normal) when Myo5 was mutant.
Previous workers have defined a sequence of comings and goings of different components at the actin patches. Some of these components are thought to move away from the plasma membrane with the endocytic vesicles in a stereotypical slow-then-fast progression. In the model based on these results, slow movement is proposed to involve invagination, and subsequent fast movement initiates only once a vesicle is pinched off and more explosive actin polymerization takes over.
Myo5 was not previously put into this sequence, but the new results suggest that Myo5 may drive vesicle pinching off (scission). The driving together of two membranes during scission could occur by directed myosin movement or contraction of an actomyosin mesh around the neck of an invaginating vesicle. Li now hopes to determine whether the motor activity of Myo5 is needed for endocytosis and to investigate Myo5 activity in an in vitro endocytosis system.