Characterization of mRNA for a proline-rich protein of cotton fiber

Cotton (Gossypium hirsutum L.) mRNA (H6) is expressed predominantly in fiber cells and is present during early primary cell wall formation. However, H6 protein is found to accumulate during later stages, when active secondary cell wall formation occurs, indicating possible regulation at the translational level and function in the secondary cell wall assembly. The nucleotide-derived amino acid sequence of pCK-H6 is proline rich (35 mol %) with a calculated molecular mass of 21 kD. Cotton protein H6 contains a repetitive pentameric motif (17) of alanine (serine)-threonine (serine)-proline-proline-proline. Its amino acid composition and solubility indicate that it may belong to the group of arabinogalactan proteins. Both sodium dodecyl sulfate-polyacrylamide gel electrophoresis (55 kD) and size-exclusion chromatography (77-83 kD) overestimated the size of in vitro synthesized H6 protein compared to the molecular mass derived from the nucleotide sequence (21 kD). The conformation of the molecule determined by its unusual primary structure may be the cause of its anomalous mobility. The presence of a proline-rich, arabinogalactan-type protein in cotton fiber raises the interesting possibility that it may be an integral part of the plasmalemma taking part in the development and architecture of the secondary wall of cotton fiber.