Regulating a translational regulator: mechanisms cells use to control the activity of the fragile X mental retardation protein

Fragile X syndrome results from the loss of a normal cellular protein, FMRP. FMRP is an RNA binding protein, and it is likely that altering the way FMRP's messenger RNA (mRNA) targets are processed results in the clinical features associated with the disease. Using complementary DNA microarray screening, a number of brain-derived mRNAs that interact directly with FMRP in vitro and associate with FMRP-containing mRNPs in vivo have been identified. These target messages encode RNA-binding proteins, transcription factors, neuronal receptors, cytoskeletal proteins, a few enzymes as well as several unknown proteins. For a subset of these mRNAs it has been shown that modulating FMRP levels in cultured cells correspondingly affects their expression. In addition, several modes by which cells modulate FMRP activity have been described; these include posttranscriptional processing and posttranslational modification. Here, the most recent results concerning the biochemical activities of FMRP and how they are affected by various modifications are reviewed. The data lead to a model signaling mechanism by which FMRP normally regulates the expression of its target mRNAs.