In most myogenic systems, synthesis of the intermediate filament (IF) protein vimentin precedes the synthesis of the muscle-specific IF protein desmin. In the dorsal myotome of the Xenopus embryo, however, there is no preexisting vimentin filament system and desmin's initial organization is quite different from that seen in vimentin-containing myocytes (Cary and Klymkowsky, 1994. Differentiation. In press.). To determine whether the organization of IFs in the Xenopus myotome reflects features unique to Xenopus or is due to specific properties of desmin, we used the injection of plasmid DNA to drive the synthesis of vimentin or desmin in myotomal cells. At low levels of accumulation, exogenous vimentin and desmin both enter into the endogenous desmin system of the myotomal cell. At higher levels exogenous vimentin forms longitudinal IF systems similar to those seen in vimentin-expressing myogenic systems and massive IF bundles. Exogenous desmin, on the other hand, formed a reticular IF meshwork and non-filamentous aggregates. In embryonic epithelial cells, both vimentin and desmin formed extended IF networks. Vimentin and desmin differ most dramatically in their NH2-terminal "head" regions. To determine whether the head region was responsible for the differences in the behavior of these two proteins, we constructed plasmids encoding chimeric proteins in which the head of one was attached to the body of the other. In muscle, the vimentin head-desmin body (VDD) polypeptide formed longitudinal IFs and massive IF bundles like vimentin. The desmin head-vimentin body (DVV) polypeptide, on the other hand, formed IF meshworks and non-filamentous structures like desmin. In embryonic epithelial cells DVV formed a discrete filament network while VDD did not. Based on the behavior of these chimeric proteins, we conclude that the head domains of vimentin and desmin are structurally distinct and not interchangeable, and that the head domain of desmin is largely responsible for desmin's muscle-specific behaviors.

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