Poster: Cytoskeleton: Structure & Function

Abs # 1118: An actin filament severing protein may function to inhibit tip growth during the self-incompatibility response of poppy pollen.

Presenter:	Staiger, Chris J., cstaiger@bilbo.bio.purdue.edu
Authors	Staiger, Chris J. (A)   Huang, Shanjin  (A)   Chaudhry, Faisal  (A)   Franklin-Tong, Noni  (B)
Affiliations:	(A): Purdue University (B): University of Birmingham, England

The actin cytoskeleton organizes the pollen cytoplasm, provides the molecular railroad tracks for cytoplasmic streaming, and orchestrates secretory vesicle delivery to the growing apex. During the self-incompability (SI) response of poppy, a cascade of intracellular signaling events is initiated to prevent pollen germination and tube growth. Central to SI is a flood of cytosolic calcium that emanates from near the male germ unit and propagates tipward, and leads to protein phosphorylation and changes in cellular architecture. The actin cytoskeleton undergoes marked reorganization and a 50–80% reduction in F-actin levels within minutes of S-protein challenge. This dramatic actin depolymerization, one of the few known examples of signal-mediated cytoskeletal disassembly, occurs within minutes and is sustained for several hours. We examined a role for profilin during SI and found that this calcium-regulated actin sequestering protein could account for only a portion of the depolymerization. We propose a model whereby the action of actin capping or severing factors enhances profilin's sequestering activity. With DNase I chromatography, we isolated an 80 kDa protein that binds to poppy actin and has calcium-stimulated actin severing and/or capping activity in vitro. Antibody cross-reactivity and MS/MS analysis identify the 80 kDa protein as a gelsolin-like actin-binding protein. Cloning of a cDNA is underway and production of a recombinant protein will provide the starting material for additional characterization of its physicochemical properties and molecular reconstitution of SI. We suggest that poppy 80 kDa is a central player in the signal-mediated rearrangement of actin filaments in pollen, and may be the first example of a plant actin filament severing activity.