The heavy chain of Acanthamoeba myosin IB is a fusion of myosin-like and non-myosin-like sequences.

Abstract

Acanthamoeba castellanU myosins IA and IB demonstrate the catalytic properties of a myosin and can support analogues of contractile and motile activity in vitro, but their single, low molecular weight heavy chains, roughly globular shapes, and inabilities to self-assemble into filaments make them structurally atypical myosins. We now present the complete amino acid sequence of the 128-kDa myosin IB heavy chain, which we deduced from the nucleotide sequence of the gene and which reveals that the polypeptide is a fusion of myosin-like and non-myosin-like sequences. Specifically, the amino-terminal =76 kDa of amino acid sequence is highly similar to the globular head sequences ofconventional myosins. By contrast, the remaining *51 kDa of sequence shows no miarity to any portion of conventional myosin sequences, contains regions that are rich in glycine, proline, and alanine residues, and lacks the distinctive sequence characteristics of an a-helical, coiled-coil structure. We conclude, therefore, that the protein is composed of a myosin globular head fused not to the typical coiled-coil rod-like myosin tail structure but rather to an unusual carboxyl-terminal domain. These results support the conclusion that filamentous myosin is not required for force generation and provide a further perspective on the structural requirements for myosin function. Finally, we find a striking conservation of intron/exon structure between this gene and a vertebrate muscle myosin gene. We discuss this observation in relation to the evolutionary origin ofthe myosin IB gene and the antiquity of myosin gene intron/exon structure.Acanthamoeba castellanii myosins IA and IB are incapable of self-assembly into bipolar filaments but, like conventional myosins, they can support analogues of contractile and motile activity in vitro (for review, see ref. 1). The only well-developed model of actomyosin-dependent contractile and motile activities is the sliding filament model, which depends specifically on the ability of myosins to selfassemble into bipolar filaments (2). Clearly, the properties of myosins IA and IB are at odds with this model. It would seem, therefore, that a full understanding ofthe structure and function of these nonfilamentous, actin-activated ATPases might provide important insights into the general mechanism of actomyosin-dependent movement. Toward this end, we present the complete amino acid sequence of the myosin IB heavy chain, which was deduced from the nucleotide sequence of the gene.* The results reveal that the myosin IB heavy chain polypeptide is a fusion of myosin-like and non-myosin-like sequences. The structure of the protein suggested by this sequence determination-i.e., a myosin globular head fused to an unusual carboxyl-terminal domain-provides a rationale for understanding the paradoxical properties of myosin I and presents a further perspective on the structural requirements for myosin function. MATERIALS AND METHODSDNA sequencing was performed by the dideoxy chaintermination method (3) and by the che...

Mammalian myosin I alpha, I beta, and I gamma: new widely expressed genes of the myosin I family.

“…3, A and B). In the cerebellum, a peak of expresNakanishi, 1987); ChkBBMI, chicken BBMI (Garcia et al, 1989); AcaMlb, Acanthamoeba myosin Ib gene (Jung et al, 1987); CrdMll, the rat alpha cardiac muscle myosin H (McNally et al, 1989). (B) Alignment of the tail sequences of MMIc~ protein and both chicken and cow BBMI.…”

“…Since the initial report of myosin I in Acanthamoeba (Pollard and Korn, 1973), the number and diversity of myosin I family members have grown. Although myosin I's have been well studied in organisms including Acanthamoeba (Jung et al, 1987;Lynch et al, 1989), Dictyostelium (Jung et al, 1989;Jung and Hammer, 1990;Titus et al, 1989), and Drosophila (Montell and Rubin, 1989;Hicks and Williams, 1992), only a single myosin I gene in vertebrates has been cloned and extensively characterized to date (Hoshimaru and Nakanishi, 1987;Garcia et al, 1989). This species appears to be localized exclusively to the brush border of the small intestine (Hoshimaru et al, 1989;Bikle et al, 1991) and has been designated brush border myosin I (BBMI).1 Recent biochemical findings have pointed to the presence of additional mammalian myosin I family members (Coluccio, 1991;Barylko et al, 1992).…”

Immunolocalization of myosin I heavy chain kinase in Acanthamoeba castellanii and binding of purified kinase to isolated plasma membranes.

“…The co-localization of myosin I heavy chain kinase and myosin I at the plasma membrane is of interest in relation to the possible functions of myosin I especially as phospholipids increase kinase activity. main closely resembles the subfragment I domain of myosin II (Jung et al ., 1987;Jung et al, 1989) . This domain contains an ATP-binding site (Lynch et al, 1987;Brzeska et al, 1988) and an ATP-sensitive actin-binding site (Brzeska et al ., 1988(Brzeska et al ., , 1989a.…”

“…When proteolytically separated from the rest of the heavy chain, the NH2-terminal domain expresses high actin-activated Mgt±ATPase activity (Brzeska et al ., 1988(Brzeska et al ., , 1989a. The COOH-terminal domain lacks the «-helical structure of the longer tail domain ofmyosin II consisting instead of a short globular tail (Jung et al, 1987) which contains an ATP-insensitive actin-binding site (Lynch et al ., 1986;Brzeska et al ., 1988) and a membrane binding site (Adams and Pollard, 1989;Miyata et al, 1989), neither of which is found in mechanoenzymes of the myosin II class.…”

New insights into the role of the cytoskeleton in phagocytosis of Entamoeba histolytica

“…Today, the unconventional myosins can be divided into at least 13 structurally distinct types of myosin heavy chains (Mermall et al, 1998). Six unconventional myosin I (A to F) have so far been identi®ed in D. discoideum (Jung et al, 1987;Urrutia et al, 1993;Peterson et al, 1995;Titus et al, 1993;1994;. The presence of myosin has been investigated in the pseudopod of motile amoebae.…”