Leishmania major: association of the differentially expressed gene B protein and the surface lipophosphoglycan as revealed by membrane capping.

The lipophosphoglycan (LPG) of Leishmania promastigotes forms a dense glycocalyx which effectively covers the entire surface of the cell, and which undergoes structural modifications during the differentiation of promastigotes to the infective or metacyclic stage. Recently, the first protein marker for metacyclic promastigotes of Leishmania major has been characterized. This protein, termed gene B protein, is located on the cell surface, yet it lacks any hydrophobic sequence for membrane attachment. It does contain an unusual amino acid repeat that is related to the peptidoglycan binding domain of protein A from Staphylococcus aureus, suggesting that the protein might interact with metacyclic LPG via this domain for attachment to the cell. We have studied the distribution of LPG, gene B protein, and the major surface protease, gp63, by labeling them with immunogold or immunofluorescence prior to and during capping events. Thin sections of double-labeled parasites revealed that the gene B protein-gold particles were colocalized with the LPG-gold particles in the LPG capping structures at the extremities of the cell. Cocapping of LPG and gene B protein was also observed with two-color fluorescence. No similar redistribution was seen in gp63 or with integral membrane proteins. In contrast to the gene B protein, gp63 could only be immunogold labeled on the metacyclic surface after capping and shedding of the LPG, providing further that it and other membrane-associated proteins are normally buried under the LPG coat. The unusual surface exposure of the gene B protein is consistent with its hydrophilic and LPG binding properties, which allow it to become incorporated into the cell coat and to localize to the most external aspects of the cell.