Identification of the Regions on the M₁₁₀ Subunit of Protein Phosphatase 1M That Interact with the M₂₁ Subunit and with Myosin

Abstract

We have previously isolated a form of protein phosphatase-I (PPIM) from avian smooth muscle myofibrils that is composed of the catalytic subunit of PP1 (PPlC) bound to an M-complex consisting Biochem. 239,. In this paper, we establish that PPIM accounts for nearly all the myosin phosphatase activity in myofibrils, that the M,,,, and M,, subunits are present at similar concentrations in the myofibrillar fraction, and that these subunits are entirely bound to PP1. We demonstrate that the M,, subunit does not interact with PPlC, but with the C-terminal 72 residues of the M,,,, subunit, a region which is 43% identical to residues 87-161 of the M,, subunit. A fragment of the M,, subunit, M,,-(Ml-L146), which lacks the C-terminal leucine zipper, also bound to the M,,, subunit, but two other fragments M2,-(M1-E110) and M,,-(E110-K186) did not. The MI ,,, and M,, subunits were both found to be myosin-binding proteins. The C-terminal 291 residues of the M,,,, subunit, but not the C-terminal 72 residues, bound to myosin, but the N-terminal fragments M,,,-(MI -E309) and M,,,-(MI-S477) did not. Thus, the region of the M,,,, subunit that stimulates the dephosphorylation of myosin by PPlC is distinct from the region that targets PPlM to myosin. Remarkably, each myosin dimer was capable of binding about 20 mol M,, subunit and many of the M,,-binding sites were located in the myosin rod domain. The potential significance of this observation is discussed.Keywords: protein phosphatase-1 (PPl) ; myosin ; smooth muscle ; muscle contraction.Protein phosphatase-I (PPl), one of the major serinelthreonine-specific protein phosphatases in eukaryotic cells, is regulated by targetting subunits that direct it to particular subcellular loci, modify its substrate specificity, and confer the ability to be regulated by extracellular signals (reviewed in [ 1,21). A significant proportion of the PP1 in vertebrate muscle extracts is associated with myofibrils and has enhanced activity towards the Plight chain of myosin and reduced activity towards other substrates, such as glycogen phosphorylase [3, 41. When isolated from avian (chicken gizzard) [4, 51 or mammalian (pig bladder) [6] smooth muscle, this form of PP1 (PPIM) was found to be composed of three subunits, namely the catalytic subunit of PP1 Note. The novel nucleotide sequence data published here have been deposited with the sequence data banks and are available under accession numbers S74907 and S74622.(PPIC) and two other proteins with molecular masses of 110 kDa and 21 kDa, termed the MI ,,, and M,, subunits, respectively [4, 51. The M,,,, subunit is complexed to both PPlC and the M,, subunit [4], and is the component that modulates the substrate specificity of PPlC because selective removal of the M,, subunit from PPIM does not affect the rate at which either myosin or glycogen phosphorylase are dephosphorylated 171.The MI ,,, subunit has been cloned from rat aorta [5], chicken gizzard [8], and rat kidney [9] cDNA libraries. The N-terminus of the M,,, subunit contains seven ankyrin r...

A Novel Regulatory Mechanism of Myosin Light Chain Phosphorylation via Binding of 14-3-3 to Myosin Phosphatase

“…Smooth-muscle myosin, MYPT1, and the 14-3-3␤ mixture was subjected to immunoprecipitation using anti-myosin antibody. MYPT1 was coimmunoprecipitated with myosin ( Figure 3C) consistent with the previous reports (Ichikawa et al, 1996;Hirano et al, 1997;Johnson et al, 1997). Quite interestingly, 14-3-3␤ markedly inhibited the binding of MYPT1 to myosin ( Figure 3C).…”

“…MYPT1 is critical to hold the three subunits together. The C-terminal 72 residues reside at the 21/20 kDa subunit binding site (Johnson et al, 1997). The catalytic subunit binds to the large subunit at two sites, a relatively strong site in the N-terminal 38 residues and a weak site in the ankyrin repeat (residues 39 -295; Hirano et al, 1997).…”

Recombinant Small Subunit of Smooth Muscle Myosin Light Chain Phosphatase

“…Whereas myosin light chain kinase and its regulation by calmodulin are well studied (3), much less was known about the phosphatase until a heterotrimeric myofibrilassociated myosin phosphatase (MP) 1 was purified from avian gizzard (4,5) and pig bladder (6). This enzyme appears to be the major if not the only myosin phosphatase in smooth muscle (7). It is composed of three subunits with apparent molecular masses (based on SDS-PAGE mobilities) of 130, 38, and 20 kDa (designated here as MPt, MPc, and MPs respectively) (reviewed in Erdödi et al (8), and Hartshorne et al (9)).…”

“…Expression of MPt fragments in Escherichia coli (17), and fulllength MPt in insect cells (18) have been reported. MPc in small amounts has been expressed in E. coli (19) and in insect cells (20), and MPs has been expressed in E. coli (7,21).Some properties of MPs have been reported. Two MPs cDNAs have been isolated, one coding for a 161-amino acid (18 kDa) protein (18,21), and the other for a 186-amino acid (21 kDa) protein containing a leucine zipper motif (16) (abbreviated as MPs (21)).…”

“…Expression of MPt fragments in Escherichia coli (17), and fulllength MPt in insect cells (18) have been reported. MPc in small amounts has been expressed in E. coli (19) and in insect cells (20), and MPs has been expressed in E. coli (7,21).…”