Actin polymerization is not absolutely required for clathrin-mediated endocytosis, and disruption of actin does not interfere with the early stages of clathrin-coated pit formation. Actin is required to complete vesicle formation under conditions of high membrane tension, such as on the apical side of polarized epithelial cell, while actin is dispensable for this process in the absence of membrane tension (Boulant et al, 2011). In cases where actin is required, it appears to be recruited late to the emerging clathrin-coated pit, just prior to or coincident with the recruitment of dynamin and vesicle scission (Taylor et al, 2011; Taylor et al, 2012; reviewed in McMahon and Boucrot, 2011). Recruitment of actin depends on the ARP2/3 complex, and cortactin or the neural Wiscott-Aldrich syndrome proteins WASL. These proteins, in turn, are recruited through interactions with N-BAR domain containing proteins such as SNX9 (Yarar et al, 2007; Shin et al, 2007; Shin et al, 2008; Ferguson et al, 2009; reviewed in Lundmark and Carlsson, 2009; McMahon and Boucrot, 2011).
HIP1 and HIP1R are additional components of the late clathrin-coated pit that interact with clathrin and AP-2 and may contribute to actin nucleation (Waelter et al, 2001; Mishra et al, 2001;Metzler et al, 2001; Legendre-Guillemin et al, 2002; Wilbur et al, 2008; Taylor et al, 2011).