THE ROLE OF SURFACE LOOPS (RESIDUE-204-216 AND RESIDUE-627-646) IN THE MOTOR FUNCTION OF THE MYOSIN HEAD

A characteristic feature of all myosins is the presence of two sequenc es which despite considerable variations in length and composition can be aligned with loops 1 (residues 204-216) and 2 (residues 627-646) i n the chicken myosin-head heavy chain sequence, Recently, an intriguin g hypothesis has been put forth suggesting that diverse performances o f myosin motors are achieved through variations in the sequences of lo ops 1 and 2 [Spudich, J, (1994) Nature (London) 372, 515-518], Here, w e report on the study of the effects of tryptic digestion of these loo ps on the motor and enzymatic functions of myosin. Tryptic digestions of myosin, which produced heavy meromyosin (HMM) with different percen tages of molecules cleaved at both loop 1 and loop 2, resulted in the consistent decrease in the sliding velocity of actin filaments over HM M in the in vitro motility assays, did not affect the V-max, and incre ased the K-m values for actin-activated ATPase of HMM, Selective cleav age of loop 2 on HMM decreased its affinity for actin but did not chan ge the sliding velocity of actin in the in vitro motility assays, The cleavage of loop 1 on HMM decreased the mean sliding velocity of actin in such assays by almost 50% but did not alter its affinity for HMM. To test for a possible kinetic determinant of the change in motility, 1-N-6-ethenoadenosine diphosphate (epsilon ADP) release from cleaved a nd uncleaved myosin subfragment 1 (S1) was examined, Tryptic digestion of loop 1 slightly accelerated the release of epsilon ADP from S1 but did not affect the rate of epsilon ADP release from acto-S1 complex, Overall, the results of this work support the hypothesis that loop 1 c an modulate the motor function of myosin and suggest that such modulat ion involves a mechanism other than regulation of ADP release from myo sin.