Adducin Preferentially Recruits Spectrin to the Fast Growing Ends of Actin Filaments in a Complex Requiring the MARCKS-related Domain and a Newly Defined Oligomerization Domain

Abstract

Adducin is a protein associated with spectrin and actin in membrane skeletons of erythrocytes and possibly other cells. Adducin has activities in in vitro assays of association with the sides of actin filaments, capping the fast growing ends of actin filaments, and recruiting spectrin to actin filaments. This study presents evidence that adducin exhibits a preference for the fast growing ends of actin filaments for recruiting spectrin to actin and for direct association with actin. beta-Adducin-(335-726) promoted recruitment of spectrin to gelsolin-sensitive sites at fast growing ends of actin filaments with half-maximal activity at 15 nM and to gelsolin-insensitive sites with half-maximal activity at 75 nM. beta-Adducin-(335-726) also exhibited a preference for actin filament ends in direct binding assays; the half-maximal concentration for binding of adducin to gelsolin-sensitive sites at filament ends was 60 nM, and the Kd for binding to lateral sites was 1.5 microM. The concentration of beta-adducin-(335-726) of 60 nM required for half-maximal binding to filament ends is in the same range as the concentration of 150 nM required for half-maximal actin capping activity. All interactions of adducin with actin require the myristoylated alanine-rich protein kinase C substrate-related domain as well as a newly defined oligomerization site localized in the neck domain of adducin. Surprisingly, the head domain of adducin is not required for spectrin-actin interactions, although it could play a role in forming tetramers. The relative activities of adducin imply that an important role of adducin in cells is to form a complex with the fast growing ends of actin filaments that recruits spectrin and prevents addition or loss of actin subunits.