ADDUCIN PREFERENTIALLY RECRUITS SPECTRIN TO THE FAST-GROWING ENDS OF ACTIN-FILAMENTS IN A COMPLEX REQUIRING THE MARCKS-RELATED DOMAIN AND ANEWLY DEFINED OLIGOMERIZATION DOMAIN

Adducin is a protein associated with spectrin and actin in membrane sk eletons of erythrocytes sind possibly other cells, Adducin has activit ies in in vitro assays of association with the sides of actin filament s, capping the fast growing ends of actin filaments, and: recruiting s pectrin 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, b eta-Adducin-(335-726) promoted recruitment of spectrin to gelsolin-sen sitive 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 co ncentration for binding of adducin to gelsolin-sensitive sites at fila ment ends was 60 nM, and the K-d for binding to lateral sites was 1.5 mu M. 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 con centration of 150 nM required for half-maximal actin capping activity. All interactions of adducin with actin require the myristoylated alan ine-rich protein kinase substrate-related domain as well as a newly de fined oligomerization site localized in the neck domain of adducin. Su rprisingly, the head domain of adducin is not required for spectrin-ac tin interactions, although it could play a role in forming tetramers, The relative activities of adducin imply that an important role of add ucin in cells is to form a complex with the fast growing ends of actin filaments that recruits spectrin and prevents addition or loss of act in subunits.