Variants of PSMA

Prostate-specific membrane antigen is alternatively spliced to produce at least three variants (Fig. 1B), most important of which is PSM', the cDNA of which is identical to PSMA except for a 266-nucleotide region near the 5' end of PSMA cDNA (nucleotides 114380), which codes for the transmembrane region of the protein. Therefore, PSM' is located in the cytoplasm. Su and co-workers (10) used RNAse protection assays to examine the expression of PSMA and PSM' in normal vs benign prostate hyperplasia (BPH) vs prostate cancers. They found increasing expression of PSMA in tumors relative to normal controls and generated a tumor index based on the PSMA : PSM' ratio, which is 9-11 in LNCaP cells, 3-6 in prostatic carcinoma, 0.75-1.6 in BPH, and 0.075-0.45 in normal prostate (Table 1). The enzymatic activity of the alternatively spliced version of PSMA : PSM', has not been undisputedly proven. Evidence in support for the cytoplasmic locations of this form in LNCaP cells has been provided by two groups (12,13). However, Barinka et al. (14) could not detect any enzymatic activity in this protein by expressing the recombinant intraellular form of this enzyme. The other two variants of PSMA are PSM-C [with transcription start site same as PSMA, splice donor site same as PSM' (nucleotide 114), different splice acceptor site located within intron 1 of PSMA gene, includes a novel exon 1b, which is identical to PSM'] and PSM-D [same donor site as PSM', but acceptor site includes novel exon 1c, has a new translation start site followed by 42 novel amino acids with a motif ala-ala-tyr-

Psma Binding

Fig. 1. (A) The globular nature of PSMA protein with different domains. Domain A: amino acids 1-19, intracellular, cytoplasmic; domain B: amino acids 20-39, transmembrane; domain C: amino acids 40-144; domain D: amino cells 173-248; domain E: amino acids 275-596; domain F: amino acids 597-756. There are two linker regions between domains C and D (amino acids 145-172) and domains D and E (amino acids 249-274). Domains C, D, E, and F are the extracellular domains. Domain E is the catalytic domain with the zinc-binding and substrate-binding sites. Cellular locations of different epitopes of different PSMA-specific monoclonal antibodies have been shown here. (B) Schematic view of the different splice variant of PSMA detected in LNCaP cells. Among the different variants of PSMA, PSM' lacks the intracellular cytoplasmic domain and transmembrane domain, the rest is identical to PSMA. PSM-C, although produced by alternative splicing, has a different splice acceptor site than PSM', encodes a protein identical to PSM'. PSM-D has a unique 42-amino-acid-long domain at the N-terminus and rest is identical to PSM'. The protein sequence is shown by the hatched box. The cDNA sequence of PSMA and its other three variants are shown by the open box. PSMA nucleotide positions are shown according to the Genbank accession number M99487.

Fig. 1. (A) The globular nature of PSMA protein with different domains. Domain A: amino acids 1-19, intracellular, cytoplasmic; domain B: amino acids 20-39, transmembrane; domain C: amino acids 40-144; domain D: amino cells 173-248; domain E: amino acids 275-596; domain F: amino acids 597-756. There are two linker regions between domains C and D (amino acids 145-172) and domains D and E (amino acids 249-274). Domains C, D, E, and F are the extracellular domains. Domain E is the catalytic domain with the zinc-binding and substrate-binding sites. Cellular locations of different epitopes of different PSMA-specific monoclonal antibodies have been shown here. (B) Schematic view of the different splice variant of PSMA detected in LNCaP cells. Among the different variants of PSMA, PSM' lacks the intracellular cytoplasmic domain and transmembrane domain, the rest is identical to PSMA. PSM-C, although produced by alternative splicing, has a different splice acceptor site than PSM', encodes a protein identical to PSM'. PSM-D has a unique 42-amino-acid-long domain at the N-terminus and rest is identical to PSM'. The protein sequence is shown by the hatched box. The cDNA sequence of PSMA and its other three variants are shown by the open box. PSMA nucleotide positions are shown according to the Genbank accession number M99487.

ala-cys-thr-gly-cys-leu-ala (found in growth factor cys-knot family of proteins) and rest of the PSMA protein in frame (1,15)] (Fig. 1B). The implication of such alternative splice variants in prostate cancer cells is not known at present.

Table 1

Relative Ratio of mRNA and Enzyme Activity of PSMA and PSMA' in Various Tissues

Table 1

Relative Ratio of mRNA and Enzyme Activity of PSMA and PSMA' in Various Tissues

PSMA/PSMA'

Normal

BPH

PCa

LNCaP

mRNA ratio

0.075-0.45

0.75-1.6

3-6

9-11

Enzyme activity/mg of protein

3.2

1.5

4.4

NDb

a Prostate cancer. b Not done.

Data from refs. 10 and 11.

a Prostate cancer. b Not done.

Data from refs. 10 and 11.

+1 0

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