PakB binds to the SH3 domain of Dictyostelium Abp1 and regulates its effects on cell polarity and early development

Dictyostelium p21-activated kinase B (PakB) phosphorylates and activates class I myosins. PakB colocalizes with myosin I to actin-rich regions of the cell, including macropinocytic and phagocytic cups and the leading edge of migrating cells. Here we show that residues 1–180 mediate the cellular localization of PakB. Yeast two-hybrid and pull-down experiments identify two proline-rich motifs in PakB-1-180 that directly interact with the SH3 domain of Dictyostelium actin-binding protein 1 (dAbp1). dAbp1 colocalizes with PakB to actin-rich regions in the cell. The loss of dAbp1 does not affect the cellular distribution of PakB, whereas the loss of PakB causes dAbp1 to adopt a diffuse cytosolic distribution. Cosedimentation studies show that the N-terminal region of PakB (residues 1–70) binds directly to actin filaments, whereas dAbp1 exhibits only a low affinity for filamentous actin. PakB-1-180 significantly enhances the binding of dAbp1 to actin filaments. When overexpressed in PakB-null cells, dAbp1 completely blocks early development at the aggregation stage, prevents cell polarization, and significantly reduces chemotaxis rates. The inhibitory effects are abrogated by the introduction of a function-blocking mutation into the dAbp1 SH3 domain. We conclude that PakB plays a critical role in regulating the cellular functions of dAbp1, which are mediated largely by its SH3 domain.