The lower cleft between domains 1 and 3 is lined by residues Tyr143, Ala144, Gly146, Thr148, Gly168, Ile341, Ile345, Leu346, Leu349, Thr351, and Met355, which are predominantly hydrophobic. This cleft constitutes the major binding site for most ABPs and also mediates important longitudinal contacts between actin subunits in the filament