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Actin filaments and microtubules are two major cytoskeletal systems involved in wide cellular processes, and the organizations of their filamentous networks are regulated by a large number of associated proteins. Recently, evidence has accumulated for the functional cooperation between the two filament systems via associated proteins. However, little is known about the interactions of the kinesin superfamily proteins, a class of microtubule-based motor proteins, with actin filaments. Here, we describe the identification and characterization of a novel kinesin-related protein named DdKin5 from Dictyostelium. DdKin5 consists of an N-terminal conserved motor domain, a central stalk region, and a C-terminal tail domain. The motor domain showed binding to microtubules in an ATP-dependent manner that is characteristic of kinesin-related proteins. We found that the C-terminal tail domain directly interacts with actin filaments and bundles them in vitro. Immunofluorescence studies showed that DdKin5 is specifically enriched at the actin-rich surface protrusions in cells. Overexpression of the DdKin5 protein affected the organization of actin filaments in cells. We propose that a kinesin-related protein, DdKin5, is a novel actinbundling protein and a potential cross-linker of actin filaments and microtubules associated with specific actin-based structures in Dictyostelium.Actin filaments and microtubules are two major cytoskeletal systems involved in a wide variety of cellular processes, including intracellular transport, cell division, cell migration, and cellular morphogenesis. These functional multiplicities result from changes in the assembly and organization of filamentous networks that are regulated by a large number of associated proteins, including cross-linking, severing, capping, and motor proteins. The kinesin superfamily is a class of microtubulebased motor proteins defined by a conserved motor domain responsible for ATP hydrolysis and microtubule binding. Since the first identification of kinesin (1), many related proteins (kinesin-related proteins, KRPs) 1 have been identified from a wide range of eukaryotic cells and shown to be involved in cell division and intracellular transports of various cargoes, including vesicles, organelles, large protein complexes, and cytoskeletal filaments (reviewed in Refs. 2 and 3).The cellular slime mold, Dictyostelium discoideum, is a useful model system to study cytoskeletons and motor proteins. It is suitable for biochemical and genetic approaches, including gene disruption and overexpression. Moreover, it shows a wide range of cellular processes, such as binary fission, intracellular transport, endocytosis, and chemotaxis. A number of studies have been made on the actin cytoskeleton and its regulation by associated proteins in Dictyostelium (reviewed in Refs. 4 and 5). In addition, some KRPs were recently identified as motors associating with membranous structures in Dictyostelium. One of them, K7, was shown to be involved in the development of Dictyostelium cells (6)....

The formation of bone occurs via a series of events that are regulated by various hormones and cytokines. We previously reported that endothelin (ET) inhibits the mineralization by osteoblastic cells and natriuretic peptide (NP) promotes osteoblastic differentiation. Therefore, we attempted to identify the genes induced by ET or NP in mouse preosteoblastic MC3T3-E1 cells using the method known as differential display-polymerase chain reaction (DD-PCR) to understand the bone metabolism further. Consequently, we found that expression levels of mRNAs for fibronectin, type XII collagen, p160 Rho-associated kinase (ROCK), caldesmon, calpain, nucleolin and a novel gene with a zinc finger motif are downregulated in osteoblasts by ET stimuli. We also found that expression levels of mRNAs for eIF-4A and a novel gene are increased by C-type natriuretic peptide (CNP) stimuli.

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