Many direct observations and indirect experimental approaches have pin-pointed two segments (sequences 1-28 and 360-372) in actin subdomain-1 which bind to myosin subfragment-1. In a previous investigation [Labbt, J. P., Mtjean, C., Benyamin, Y. & Roustan, C. (1990) Biochem. J. 271, 407-4131, we have observed competition between myosin subfragment-1 and anti-actin antibodies specific to epitopes including Thrl03. A multisite interface model has also been proposed to take into account myosin-head binding to the N-terminal and C-terminal regions and to more central 40 -11 3 sequence of actin.In the present study, two limited actin segments encompassing residues 96-103 and 112-125 were identified as myosin-head-binding sites. The in vitro inhibitory effect of filamin on actin-activated Mg*+-ATPase would thus be explained by this competition. Furthermore, the (27-ma-50-kDa-20-kDa) trypsin-split myosin subfragment-1 which could no longer be activated by actin, did not bind at all to the two sites located in the 96-125 region, but it still interacted with the 360-372 segment.Our results regarding the position of the myosin head on actin monomers in rigor conditions provide evidence on the presence of two topologically independant contact points in the myosinheaaactin interface. One group exposed residues in the 1-7, 21-29, 77-95 and 96-103 actin segment, another, on the opposite side of subdomain-1. included residues from 112-125 and 360-372 sequences.Identification of the actidmyosin interface is essential in understanding the cyclical enzymic and mechanical process of myofibrillar contraction. Several indirect experimental approaches such as chemical cross-linking (Sutoh, 1982), NMR Correspondence to