Identity of Pltgp40 and lysomal integral membrane protein-CD63.

To the Editor: broad protein band between 30 and 60 Kd due to the high glycosylation. Hildreth et al’ reported a similar pattern for Pltg40. Recently we have cloned LIMP-CD637 nucleotide sequence analyses indicated that it consists of 238 amino acids, with four transmembrane regions and three N-glycosylation sites. The predicted molecular weight of the protein is 25 Kd. The reported N-terminus sequence of Pltgp40 agrees with the sequence of LIMP-CD63. This finding suggests that Pltg40 and LIMP-CD63 are highly homologous or identical. Recently, Hildreth et al’ characterized a 40-Kd platelet glycoprotein designated Pltgp4O. The protein was detected on activated but not on resting platelets. We would like to point out that several data suggest that Pltg40 is similar to the 53-Kd glycoprotein (lysosomal integral membrane protein [LIMP]-CD63) that we previously described.’ LIMP-CD63 is a lysosomal granule membrane protein that is expressed on the platelet plasma membrane after platelet activation. The protein was characterized with the antibody 2.28, an antibody that was clustered as CD63 at the IVth International Workshop on Leucocyte antigen^?.^ Pltg40 and LIMP-CD63 are both platelet activation markers and both have a wide cell distribution. In immunoprecipitation studies, LIMP-CD63 gives a MARCEL J. METZELAAR H. KAREL NIEUWENHUIS Department of Haematology University Hospital Utrecht Utrecht, the Netherlands