Location of the binding site of the mannose-specific lectin comitin on F-actin

We have used electron microscopy and computer image processing to produce a three-dimensional reconstruction of F-actin filaments decorated with the p utative lectin and actin-binding protein comitin. These reconstructions sho w that comitin binds to F-actin at high radius primarily to actin subdomain 1. This location is distinctly different from the binding site on F-actin for other actin bundling proteins, such as members of the a-actinin family, and may result from the positively charged comitin interacting with negati vely charged sites near the actin N terminus in subdomain 1. The location o f the comitin binding site and its restriction to subdomain 1 on a single a ctin monomer is consistent with comitin's having a function distinct from o ther actin-binding proteins and, for example, would enable comitin to link bundled actin filaments to the Golgi. (C) 1998 Academic Press.