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Comitin

A 24kDa ABP from Dictyostelium (Stratford & Brown,1985; Noegel et al, 1990) but subsequently detected  in mammalian cells (Weiner et al, 1993).  The protein is composed of two domains, a 144 amino-acid N-terminal domain, and a 41 amino-acid C-terminal domain having five (Noegel et al, 1990), or six (Weiner et al, 1993),  repeats containing GYPPQP -like sequences. These repeats are also found in the annexin family, and the repeat motif of comitin has been used as a probe to pull out annexin members from Dictyostelium (Döring et al, 1991).  Comitin increases the viscosity of F-actin solutions (Weiner et al, 1993), indicating that either the protein oligimerises or that it contains two actin binding sites. Comitin has been found  to be a component of the Golgi apparatus (Weiner et al, 1993), in both Dictyostelium and mammalian cells and is phosphorylated on both serine and tyrosine, but as yet the significance of this phosphorylation is not known.  Comitin is unusual in that the site on actin (Fulgenzi et al, 1998), close to the N-terminus of actin but distinct to many other ABPs so that actin can bind comitin and other ABPs.  Consistent with this unusual actin binding habit is the fact that comitin's actin binding  motif  is thus far unique, not showing sequence homology with any other ABP (Jung et al, 1996).  The actin-binding region of comitin has been determined to reside between amino-acids 90-135.

MGHGVRGHYSMKLRSSGGQIVVYDKYKQILYSSKPCTRDHLLSLPCA

 The consequence of the position of the actin binding site in sub-domain 1, is that comitin binds at high radius (i.e it sticks out).  Other proteins such as cofilin that binds close to this region have a low radius, but EF-1a has some shared characteristics with comitin in its actin binding habit and characteristics.  As has been pointed out (Fulgenzi et al, 1998), EF-1a also binds actin at high radius (Owen et al, 1992), and both ABPs bind G-actin as well as F-actin (Jung et al, 1996)(see EF-1a).

    The sequence of comitin does not contain a recognizable signal sequence and is not thought to be a transmembranous protein (Noegel et al, 1990) .  However, comitin is firmly associated with membranes (Jung et al, 1996, Weiner et al, 1993) so that no cytosolic pool of comitin is detectable (Weiner et al, 1993).  It is now clear that comitin is a mannose-specific lectin (Jung et al, 1996), exactly how the comitin molecule binds any mannose residues is not certain, but it is known that glycosylation does occur on both sides of membranes so it is possible that golgi-specific glyco-proteins bearing mannose sugars are exposed at the cytoplasmic face available for comitin binding.  Genes that encode lectins and other activities are known, for example the stinging nettle lectin is also a chitinase (Lerner & Raikhel, 1992).

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References

Döring, V., M. Schleicher, et al. (1991). "Dictyostelium annexin VII (synexin)." J.Biol.Chem. 266(26), 17509-17515.

Owen, C.H, DeRosier, D.J., & Condeelis, J.A. (1992). "Actin crosslinking protein EF1a of Dictyostelium discoideum has a unique bonding rule that allows square-packed bundles." J.Struct.Biol. 109, 248-254.

Fulgenzi, G., Graciotti, L., Granata, A.L., Corsi, A., Fucini, P., Noegel, A.A., Kent, H. & Stewart, M. (1998). “Location of the binding site of the mannose-specific lectin comitin on F-actin.” J.Mol.Biol. 284, 1255-1263.

Jung, E., Fucini, P., Stewart, M., Noegel, A.A. & Schleicher, M. (1996). "Linking microfilamnets to intracellular membranes: the actin binding and vesicles-associated protein comitin exhibits a mannose-specific lectin activity." EMBO J. 15, 1238-1246.

Lerner, D. R. & Raikhel, N.V. (1992). “The gene for stinging nettle lectin (Urtica dioica Agglutinin) encodes both a lectin and a chitinase.” J.Biol.Chem. 267, 11085-11091.

Noegel, A. A.,  Gerisch, G., Lottspeich, F. & Schleicher, M.  (1990). “A protein with homology to the C-terminal repeat sequence of Octopus rhodopsin and synaptophysin is a member of a multigene family in Dictyostelium discoideum.” FEBS letters 266, 118-122.

Ponte, E., Rivero, F., Fechheimer, M., Noegel, A.A., & Bozzaro, S. (2000). "Severe developmental defects in Dictyostelium discoidium null mutants for actin-binding proteins." Mech.Development 91, 153-161.

Stratford, C.A. & Brown, S.S. (1985). "Isolation of an actin-binding protein from membranes of Dictyostelium discoideum."  J.Cell Biol. 100, 727-735.

Weiner, O. H., Murphy, J., Griffiths, G., Schleicher, M. & Noegel, A.A.(1993). “The actin-binding protein comitin (p24) is a component of the Golgi apparatus.” J. Cell Biol 123(1),  23-24.

Mohrs MR. Janssen K-P. Kreis T. Noegel AA. Schleicher M. (2000)."Cloning and characterization of beta-COP from Dictyostelium discoideum." Eur. J.Cell Biol. 79(5),350-357

Ugur O. Jones TLZ. (2000)."A proline-rich region and nearby cysteine residues target XLalphas to the Golgi complex region." Molecular Biology of the Cell. 11(4), 1421-1432

Ponte E. Rivero F. Fechheimer M. Noegel A. Bozzaro S. (2000)."Severe developmental defects in Dictyostelium null mutants for actin- binding proteins. Mech.Develop. 91(1-2),153-161

 
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