Abstract
SCAB1 is a novel plant-specific actin-binding protein that binds, bundles, and stabilizes actin filaments and regulates stomatal movement. Here, we dissected the structure and function of SCAB1 by structural and biochemical approaches. We show that SCAB1 is composed of an actin-binding domain, two coiled-coil (CC) domains, and a fused immunoglobulin and pleckstrin homology (Ig-PH) domain. We determined crystal structures for the CC1 and Ig-PH domains at 1.9 and 1.7 Å resolution, respectively. The CC1 domain adopts an antiparallel helical hairpin that further dimerizes into a four-helix bundle. The CC2 domain also mediates dimerization. At least one of the coiled coils is required for actin binding, indicating that SCAB1 is a bivalent actin cross-linker. The key residues required for actin binding were identified. The PH domain lacks a canonical basic phosphoinositide-binding pocket but can bind weakly to inositol phosphates via a basic surface patch, implying the involvement of inositol signaling in SCAB1 regulation. Our results provide novel insights into the functional organization of SCAB1.
Highlights
SCAB1 is a novel plant-specific actin-bundling protein
We have shown that SCAB1 has a modular organization and comprises the actinbinding domain (ABD), CC1, CC2, and immunoglobulin and pleckstrin homology (Ig-PH) domains
We have refined the boundaries of the ABD to a small region spanning residues 54 –100 and identified a few residues in this region that are important for actin binding
Summary
SCAB1 is a novel plant-specific actin-bundling protein. Results: SCAB1 consists of an actin-binding domain, two coiled-coil dimerization domains, and a fused immunoglobulin and pleckstrin homology domain with an atypical binding site for inositol phosphates. SCAB1 is a novel plant-specific actin-binding protein that binds, bundles, and stabilizes actin filaments and regulates stomatal movement. Function of the actin cytoskeleton are regulated by numerous actin-binding proteins (ABPs) that affect AF polymerization, nucleation, capping, depolymerization, severing, bundling, and interaction with other cellular components [1,2,3]. Through interaction with numerous phosphoinositidebinding domains, phosphoinositides are involved in AF assembly on specific membrane areas, signal transduction, and regulation of the activity of a large number of actinbinding/modulating proteins. It is largely unknown whether phosphoinositides regulate the formation and maintenance of actin bundles in plants. These results provide substantial novel insights into the functional organization of SCAB1
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