Annexins as regulators of invertebrate phototransduction

Two proteins from crayfish (Orconectes limosus) retinae bind Ca(2+)-dependently to phospholipid membranes and crossreact with an antibody against calelectrin (annexin IV). These proteins (p48 and p40) are prominent substrates for the protein carboxyl methyl transferase (PCMT) which we propose to be an intracellular protein crosslinking enzyme. Methylation and consequent crosslinking of p40 and p48 to the cytoskeleton or to the plasma membrane seem to be regulated by phosphorylation. In vivo inhibition of the PCMT abolished reversibly the phototransduction in Limulus ventral photoreceptors and solubilized the rhodopsin from the cortical cytoskeleton of the microvilli. We propose a model showing the annexins to regulate the organization of the microvillar cytoskeleton the integrity of which in turn is essential for an unhindered phototransduction.