Identification of functional PDZ domain binding sites in several human proteins

TIP-15 was previously identified as a cellular protein that can bind to the C-terminal end of the HTLV-1 Tax protein via its two PDZ domains. The sequ ence of the N-terminal part of TIP-15 is identical to that of the synaptic protein PSD-95. Both proteins are likely to be produced from the same gene by alternative splicing. Whereas expression of the PSD-95 mRNA was detected only with brain RNAs, that of TIP-15 was detected with RNAs from thymus, b rain, skeletal muscle and Jurkat cells. The TIP-15 protein exhibits an appa rent molecular weight of 40 kD and is weakly expressed in T cell lines. A t wo-hybrid screen performed with TIP-15 as bait revealed the presence of a P DZ binding site (PDZ-BS) in the following proteins: Lysyl tRNA synthetase, 6-phosphogluconolactonase (6-GPL), Stress-activated protein kinase 3 (SAPK3 ), NET-1, Diacylglycerol kinase zeta, MTMR1, MCM7, and hSec8. The sequence at the C-terminal ends of these proteins matches the X-S/T-X-V-COOH consens us previously defined for PDZ-BSs, with the exception of 6-GPL and SAPK3 wh ich include a leucine as the C-terminal residue. For Lysyl tRNA synthetase, NET1, MTMR1 and hSec8, binding to TIP-15 was confirmed by co-immunoprecipi tation experiments performed with the extracts of transfected COS7 cells. T hese results show the existence of functional PDZ-BSs in these proteins, bu t future studies will be necessary to establish whether or not TIP-15 repre sents a physiological partner. The significance of the presence of a PDZ-BS in these various proteins is discussed with respect to their function.