Purification and Characterization of a Smooth Muscle Myosin Light Chain Kinase-Phosphatase Complex

Abstract

We show that a myofibrillar form of smooth muscle myosin light chain phosphatase (MLCPase) forms a multienzyme complex with myosin light chain kinase (MLCKase). The stability of the complex was indicated by the copurification of MLCKase and MLCPase activities through multiple steps that included myofibril preparation, gel filtration chromatography, cation (SPSepharose BB) and anion (Q-Sepharose FF) exchange chromatography, and affinity purification on calmodulin and on thiophosphorylated regulatory light chain columns. In addition, the purified complex eluted as a single peak from a final gel filtration column in the presence of calmodulin (CaM). Because a similar MLCPase is present in varying amounts in standard preparations of both MLCKase and myosin filaments, we have named it a kinase-and myosin-associated protein phosphatase (KAMPPase).The KAMPPase multienzyme complex was composed of a 37-kDa catalytic ( Phosphorylation of myosin by myosin light chain kinase (MLCKase) 1 represents the key activation step leading to contraction of smooth muscle (for reviews, see Refs. 1-4). Relaxation or inactivation of myosin is accomplished by a myosin light chain phosphatase (MLCPase) that has a controversial identification and subunit composition (see Ref. 5). In numerous previous studies (for references, see Ref. 6), many types of cytosolic MLCPases have been purified exhibiting different specific activities toward phosphorylated myosin or isolated myosin regulatory light chain (ReLC). A common feature of all of these phosphatases seems to be the presence of not only a catalytic (PC) subunit of about 35-38 kDa but also another subunit in the range of 55-72 kDa. The function of the latter subunit has not been established. Initial attempts to classify these serine/threonine phosphatases were not very successful (7), and it appears that smooth muscle MLCPases could be either the PP1 or the PP2A type.Several years after our initial report on the first myofibrillar MLCPase (8), we and others again turned our attention to MLCPases from smooth muscle. In our new approach, the phosphatase was purified by CaM affinity chromatography and was shown to be composed of 37-kDa catalytic and 67-kDa targeting subunits (9). The only other myofibrillar phosphatase known so far was purified and partly characterized by Alessi et al. (Ref. 10; see also Refs. 11 and 12). It is composed of three subunits: a 37-kDa catalytic subunit and two regulatory subunits of 130 and 20 kDa. Although the sequence of all three subunits has been determined, the role of the regulatory subunits is not understood (see Ref. 5). In this report, we describe further results on our myofibrillar smooth muscle protein phosphatase, which is closely associated with MLCKase and myosin filaments and is called, therefore, a kinase-and myosin-associated protein phosphatase (KAMPPase). We show for the first time that this association results in a functional multienzyme complex between these two key regulatory enzymes of smooth muscle.

Purification and Characterization of a Kinase-associated, Myofibrillar Smooth Muscle Myosin Light Chain Phosphatase Possessing a Calmodulin-targeting Subunit

“…Its relation to the only other myofibrillar phosphatase of Alessi et al (38) is discussed in the companion paper (15), and it is only briefly considered here (see below). A purified aortic smooth muscle myosin phosphatase, composed of two subunits of 67 and 37 kDa (39), is structurally similar to the myosin phosphatase from cardiac muscle (40).…”

“…This subunit possessed all of the phosphatase activity. The role of the 67-kDa subunit was established as the CaM-targeting one because it was responsible for binding the PC subunit to the CaM affinity gel; the most illustrative data is presented in the companion paper (15).…”

“…The KAMPPase was purified as a PTC holoenzyme composed of the two subunits or as a large multienzyme complex, which included MLCKase and CaM (15). In addition, a homogenous preparation of the PC catalytic subunit was obtained when the targeting PT subunit was degraded during purification.…”

“…Because the apparent very tight association of the MLCPase activity with the kinase also was observed with MLCKase bound to myosin filaments (see below), we refer to this MLCPase as the kinase-and myosin-associated protein phosphatase (KAMPPase). The properties of the complex formed between the two key regulatory enzymes of smooth muscle contraction is the subject of the companion report (15). The present article focuses on the characteristics of the KAMPPase holoenzyme (PTC) and its isolated catalytic subunit (PC).…”

“…In the present report, we characterize a holoenzyme of this MLCPase, which, as we show, is bound to calmodulin in a Ca 2ϩ -independent manner. Formation of a complex between the holoenzyme and MLCKase is addressed in the accompanying report (15). 32 P]ATP was purchased from DuPont NEN and diluted with cold ATP of special grade (catalog number 519 987; Boehringer Mannheim) to the required specific activity and concentration (16).…”