SR and SR-related proteins have been implicated as trails-acting factors that play an important role in splice selection and are involved at specific stages of spliceosome formation. A well-established property of SR protein splicing factors is their ability to influence selection of alternative splice sites ill a concentration-dependent manner. Identification Of Molecules that regulate SR family protein expression is therefore of vital importance in RNA biology. Here we report that depletion of Pnn expression, a SR-related protein with functions involved in pre-mRNA splicing and mRNA export, induces reduced expression of a subset Of cellular proteins, especially that f SR family proteins, including SC35, SRm300, SRp55, and SRp40, but not that of other nuclear proteins, Such Lis p53, Mdm2, and ki67. Knocking down Pnn expression was achieved in vitro by siRNA transfection. Expression levels of SR and SR-related proteins in Pnn-depleted cells as compared to those in control cells were evaluated by immnofluorescent staining and Western blot with specific antibodies. In addition, we also demonstrate that loss of Pnn expression Could modulate splice site selection of model reporter gene ill vivo. Our finding is significant in terms of regulation of SR protein cellular concentration because it reveals that Pnn may play a general role in the control of the cellular amount Of family SR proteins through down-regulation of its own expression, thereby providing its with a better understanding of the Cellular mechanism by which Pull fulfills its biological function. (c) 2006 Elsevier Inc. All rights reserved.