Abstract
Cyclase-associated protein (CAP) is a conserved actin-binding protein that regulates multiple aspects of actin dynamics, including polymerization, depolymerization, filament severing, and nucleotide exchange. CAP has been isolated from different cells and tissues in an equimolar complex with actin, and previous studies have shown that a CAP–actin complex contains six molecules each of CAP and actin. Intriguingly, here, we successfully isolated a complex of Xenopus cyclase-associated protein 1 (XCAP1) with actin from oocyte extracts, which contained only four molecules each of XCAP1 and actin. This XCAP1–actin complex remained stable as a single population of 340 kDa species during hydrodynamic analyses using gel filtration or analytical ultracentrifugation. Examination of the XCAP1–actin complex by high-speed atomic force microscopy revealed a tripartite structure: one middle globular domain and two globular arms. The two arms were observed in high and low states. The arms at the high state were spontaneously converted to the low state by dissociation of actin from the complex. However, when extra G-actin was added, the arms at the low state were converted to the high state. Based on the known structures of the N-terminal helical-folded domain and C-terminal CARP domain, we hypothesize that the middle globular domain corresponds to a tetramer of the N-terminal helical-folded domain of XCAP1 and that each arm in the high state corresponds to a heterotetramer containing a dimer of the C-terminal CARP domain of XCAP1 and two G-actin molecules. This novel configuration of a CAP–actin complex should help to understand how CAP promotes actin filament disassembly.
Highlights
Regulated assembly and disassembly of actin filaments are vital to the diverse function of the actin cytoskeleton [1]
When Xenopus oocyte extracts were applied to a column in which glutathione S-transferase (GST)-fused Xenopus ADF/cofilin (XAC) was immobilized, several proteins bound to the column as described previously [35] (Fig. 1A)
We attempted to isolate Xenopus cyclaseassociated protein 1 (XCAP1) using anion-exchange chromatography followed by hydroxyapatite chromatography, but XCAP1 and actin were not separated during these procedures and were instead purified together in an equimolar ratio (Fig. 1B)
Summary
Regulated assembly and disassembly of actin filaments are vital to the diverse function of the actin cytoskeleton [1]. We purified a native complex of CAP and actin from Xenopus oocyte extracts (Fig. 1). There were no detectable peaks that corresponded to dissociated XCAP1 or actin, indicating that the XCAP1–actin complex was stable during the SEC-MALS analysis.
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