GRIP1 mediates the interaction between the amino- and carboxyl-termini of the androgen receptor

The androgen receptor (AR) mediates transactivation of target genes by acting as a dimer in which its amino-terminal domain (AR-NTD) interacts with its carboxyl-terminal, ligand-binding domain (AR-LBD) (N/C interaction). Here we assessed if and how AR N/C interaction relates to AR transactivation activity and how the p160 coactivator GRIP1 participates in both processes. The concentration of dihydrotestosterone needed for half-maximal N/C interaction was approximately 10-fold higher than for half-maximal transactivation, indicating a disparity between the two processes. Although a mutation of an LXXLL-like motif, 23 FQNLF 27 --> 23 FQNAA 27 , in the AR-NTD abolished AR N/C interaction, it could be restored by the co-expression of the coactivator GRIP1. Co-expression of mutated forms of GRIP1, possessing alterations known to abolish either of the two AR interaction domains, could not restore AR N/C interaction, suggesting that wild-type GRIP1 normally bridges the two AR domains. Although AR transactivation activity can proceed without AR N/C interaction, we propose that part of the GRIP1 coactivation activity resides in its ability to bind both AR-NTD and -LBD, to stabilize the N/C complex and allow for secondary cofactors to be recruited more efficiently. Our results also indicate that AR N/C interaction enhances but is not necessary for AR transactivation activity.     

The function of TIF2/GRIP1 in mouse reproduction is distinct from those of SRC-1 and p/CIP

Human TIF2 (hTIF2) is a member of the p160 family of nuclear receptor coactivators, which includes SRC-1 and p/CIP. Although the functions of hTIF2 and of its mouse homolog (GRIP1 or mTIF2) have been clearly established in vitro, their physiological role remains elusive. Here, we have generated mice lacking mTIF2/GRIP1 and examined their phenotype with a particular emphasis on reproductive functions. TIF2(-/-) mice are viable, but the fertility of both sexes is impaired. Male hypofertility is due to defects in both spermiogenesis (teratozoospermia) and age-dependent testicular degeneration, and TIF2 expression appears to be essential for adhesion of Sertoli cells to germ cells. Female hypofertility is due to a placental hypoplasia that most probably reflects a requirement for maternal TIF2 in decidua stromal cells that face the developing placenta. We conclude that TIF2 plays a critical role in mouse reproductive functions, whereas previous reports have not revealed serious fertility impairment in SRC-1(-/-) or p/CIP(-/-) mutants. Thus, even though the three p160 coactivators exhibit strong sequence homology and similar activity in assays in vitro, they play distinct physiological roles in vivo, as their genetic eliminations result in distinct pathologies.     

Interaction between GRIP and liprin-alpha/SYD2 is required for AMPA receptor targeting

Interaction with the multi-PDZ protein GRIP is required for the synaptic targeting of AMPA receptors, but the underlying mechanism is unknown. We show that GRIP binds to the liprin-alpha/SYD2 family of proteins that interact with LAR receptor protein tyrosine phosphatases (LAR-RPTPs) and that are implicated in presynaptic development. In neurons, liprin-alpha and LAR-RPTP are enriched at synapses and coimmunoprecipitate with GRIP and AMPA receptors. Dominant-negative constructs that interfere with the GRIP-liprin interaction disrupt the surface expression and dendritic clustering of AMPA receptors in cultured neurons. Thus, by mediating the targeting of liprin/GRIP-associated proteins, liprin-alpha is important for postsynaptic as well as presynaptic maturation.