Selective localization of myosin‐I proteins in macropinosomes and actin waves

Abstract

Class I myosins are widely expressed with roles in endocytosis and cell migration in a variety of cell types. Dictyostelium express multiple myosin Is, including three short-tailed (Myo1A, Myo1E, Myo1F) and three long-tailed (Myo1B, Myo1C, Myo1D). Here we report the molecular basis of the specific localizations of short-tailed Myo1A, Myo1E and Myo1F compared to our previously determined localization of long-tailed Myo1B. Myo1A and Myo1B have common and unique localizations consistent with the various features of their tail region; specifically the BH sites in their tails are required for their association with the plasma membrane and heads are sufficient for relocalization to the front of polarized cells. Myo1A does not localize to actin waves and macropinocytic protrusions, in agreement with the absence of a tail region which is required for these localizations of Myo1B. However, in spite of the overall similarity of their domain structures, the cellular distributions of Myo1E and Myo1F are quite different from Myo1A. Myo1E and Myo1F, but not Myo1A, are associated with macropinocytic cups and actin waves. The localizations of Myo1E and Myo1F in macropinocytic structures and actin waves differ from the localization of Myo1B. Myo1B colocalizes with F-actin in the actin waves and at the tips of mature macropinocytic cups whereas Myo1E and Myo1F are in the interior of actin waves and along the entire surface of macropinocytic cups. Our results point to different mechanisms of targeting of short- and long-tailed myosin Is, and are consistent with these myosins having both shared and divergent cellular functions.

A plasma membrane template for macropinocytic cups

“…In support of the genetic evidence, myosin-1 isoforms are recruited to macropinocytic cups in both Dictyostelium and Acanthamoeba (Brzeska et al, 2012; Ostap et al, 2003), most likely due to their affinity for PIP3 (Chen et al, 2012). The PIP3-binding MyoE and MyoF are recruited in the centre and MyoB at the periphery of macropinocytic cups, forming a striking ‘bull’s eye’ pattern (Brzeska et al, 2016; Dieckmann et al, 2010). In such a scenario, CARMIL may provide the link between myosin-1 and SCAR (Jung et al, 2001).…”

Uses and abuses of macropinocytosis

“…Notably, the protein kinase Akt (encoded by pkbA) and a set of myosin-I motor proteins are recruited by PIP3 to macropinosomes in Dictyostelium, and their genetic deletion strongly impairs macropinocytosis (Rupper et al, 2001;Chen et al, 2012). In Dictyostelium, myosin-IB, which does not bind to PIP3, is also recruited to the macropinosome rim, where it forms a striking 'bull's eye' pattern with the PIP3-binding proteins (Brzeska et al, 2016). Because Ras activity can control the size of macropinosomes, it is tempting to speculate that the Ras and/or PIP3 patches play a direct organizational role in macropinosome formation by delimiting a membrane region that is marked as active and operative for macropinocytosis (Fig.…”

ADiaphanous-related formin links Ras signaling directly to actin assembly in macropinocytosis and phagocytosis

“…The latter process is associated with rapid disassembly of the F-actin coat (18), and, consistently, a number of F-actin depolymerization factors, including coronin, actin-interacting protein (Aip1), and cofilin, were shown to participate in cup maturation and disassembly (18)(19)(20). Moreover, force generation by myosins was shown to play an important role in amoeba as well as in mammalian cells (17,21,22). However, it still remains elusive how Ras and PIP 3 signaling are linked to actin assembly on the molecular level to drive extensive membrane deformations during cup formation.…”