Characterization of Yeast Yea4p, a Uridine Diphosphate-N-acetylglucosamine Transporter Localized in the Endoplasmic Reticulum and Required for Chitin Synthesis*

Chitin is an essential cell wall component, synthesis of which is regulated throughout the cell cycle in the yeastSaccharomyces cerevisiae. We cloned an S. cerevisiaegene, YEA4, whose product is homologous to the Kluyveromyces lactisuridine diphosphate-N-acetylglucosamine (UDP-GlcNAc) transporter. An epitope-tagged Yea4p localized mainly in the 10,000 ×gpellet (P2), suggesting endoplasmic reticulum (ER) localization. Membrane vesicles from the P2 fraction showed an 8-fold higher UDP-GlcNAc transport activity in cells harboring a multicopyYEA4plasmid than in cells harboring vector alone. The activity distribution is identical with the protein distribution in P2, whether the gene is overexpressed or not, suggesting its native localization in P2. Immunolocalization of epitope-tagged Yea4p further revealed ER localization. The increase in transport activity due to theYEA4overexpression is specific for UDP-GlcNAc, but not for UDP-galactose and GDP-mannose. Δyea4-disrupted cells showed a reduced rate of UDP-GlcNAc transport, contained less chitin, and were larger and rounder in shape than the wild type cells. Our results indicate that YEA4encodes an ER-localized UDP-GlcNAc transporter that is required for cell wall chitin synthesis in S. cerevisiae.