Phosphorylation-dependent Structural Changes in the Regulatory Light Chain Domain of Smooth Muscle Heavy Meromyosin

Abstract

Smooth muscle heavy meromyosin, a double-headed proteolytic fragment of myosin lacking the COOH-terminal two-thirds of the tail, has been shown previously to be regulated by phosphorylation. To examine phosphorylation-dependent structural changes near the head-tail junction, we prepared five well regulated heavy meromyosins containing single-cysteine mutants of the human smooth muscle regulatory light chain labeled with the photocross-linking reagent, benzophenone-iodoacetamide. For those mutants that generated cross-links, only one type of cross-linked species was observed, a regulatory light chain dimer. Irradiated mutants fell into two classes. First, for Q15C, A23C, and wild type (Cys-108), a regulatory light chain dimer was formed for dephosphorylated but not thiophosphorylated heavy meromyosin. These data provide direct chemical evidence that in the dephosphorylated state, Gln-15, Ala-23, and Cys-108 on one head are positioned near (within 8.9 Å) the regulatory light chain of the partner head and that thiophosphorylation abolishes proximity. This behavior was also observed for the Q15C mutant on a truncated heavy meromyosin lacking both catalytic domains. For the actin-heavy meromyosin complex, cross-links were formed in both de-and thiophosphorylated states. S59C and T134C mutants were in a second mutant class, where regulatory light chain dimers were not detected in dephosphorylated or thiophosphorylated heavy meromyosin, suggesting positions outside the region of interaction of the regulatory light chains.The actin-activated ATPase activity and motor properties of smooth muscle and nonmuscle myosins are regulated by phosphorylation of the regulatory light chain (1-3). The dephosphorylated forms of these regulated myosins have low ATPase activity and are unable to move actin filaments, whereas phosphorylated forms are activated in both respects. Domain requirements for regulation have been elucidated through studies of various proteolytic and expressed subfragments of SMM. 1 SMM contains two head domains (S1) attached to a long ␣-helical coiled-coil domain (tail). Single-headed myosin (4) and S1 (5, 6) are active in both dephosphorylated and phosphorylated states. HMM that lacks the COOH-terminal two-thirds of the tail is double-headed and well regulated (5, 7), but expressed HMMs with shorter tails failed to form double-headed structures and were found to be unregulated (8, 9) as in S1 and single-headed myosin. Therefore, two heads are critical for down-regulation, suggesting that head-head interaction is an important feature of the dephosphorylated state. However, a double-headed structure may not be sufficient for full regulation, and interactions between heads and rods may also be required (10).The general location of the phosphorylated subunit (RLC) at the junction between the heads and tail (11) suggests that RLC-RLC interactions may be a logical consequence of headhead interactions. Indeed, the RLC has been shown to be critical to the regulatory mechanism. In particular, disruption of the COOH-ter...

Novel Sensors of the Regulatory Switch on the Regulatory Light Chain of Smooth Muscle Myosin

“…HMM and S1 were prepared from a V8 digest of SMM (22). Protein concentrations were determined using the following extinction coefficients: SMM, ⑀ 280 0.1% ϭ 0.56; HMM, ⑀ 280 0.1% ϭ 0.65; S1, ⑀ 280 0.1% ϭ 0.75.…”

“…Myosin light chain kinase was prepared from frozen chicken gizzards (27) with modifications (6). Human smooth muscle RLC (MLRN_HUMAN, accession P24844) mutants were expressed and purified from Escherichia coli BL21 (DE3) (22).…”

“…RLC Exchange and His Tag Removal-RLC was exchanged onto either HMM or S1 (22,30), and unbound RLC was removed by Superdex 200 gel filtration (Amersham Biosciences). Gel densitometry (6) indicated a 30 -80% exchange efficiency.…”

“…Exchange efficiency for thiol reactivity experiments was 70 -100% for HMM and Ն95% for S1. Samples were precipitated with (NH 4 ) 2 SO 4 (70% saturation) and dialyzed into 10 mM MOPS (pH 7.0), 1 mM DTT, 50 mM NaCl, 0.2 mM EGTA, and the His tag was cleaved by thrombin treatment (22), leaving the sequence GSH before the N-terminal Met.…”

“…Unfortunately, none of the crystal structures contain coordinates for the RLC N terminus (residues 1-24), presumably due to its lack of ordered secondary structure in single-head constructs. Our photocross-linking studies (22,23) provide structural information about the RLC N terminus in UP-HMM. Our data (23) suggest that phosphorylation of Ser-19 causes a large movement of the N terminus from an extended to a folded conformation.…”