Formation and possible roles of specific phospho proteins

Formation and possible roles of specific (phospho)proteins 

With regard to the possible roles of the detected phosphoproteins in steroidogenesis, the phosphorylation of the nuclear 17 kDa protein occurs too slowly for it to be directly involved. It may be involved in regulation of gene expression and consequently the trophic effects of LH. It has been shown that RNA synthesis is not required in the acute stimulation of steroidogenesis [43,41]. 

Rapid phosphorylation of the other detected phosphoproteins does occur but no definite roles have yet been ascribed to them. The 33 kDa protein may be the S6 ribosomal protein involved in the control of protein synthesis. The 57 kDa protein has been identified as the regulatory suhunit of the cyclic AMP-dependent protein kinase [44]. Of the other proteins the 76, 43 and 20 kDa may be connected with the microfilaments (43 kDa actin, 76 kDa myosin light chain kinase and 20 kDa myosin light chain) but this must be further investigated. These proteins may only play a permissive role in. steroidogenesis. The fact that the pattern of protein phosphorylation is very similar after stimulation of protein kinase C with phorbol esters supports this because the latter only marginally increase steroidogenesis [18]. 

The stimulation of steroid synthesis in gonadal cells is dependent on ongoing protein synthesis. Experiments with protein synthesis inhibitors have shown the presence of one or more proteins with half lives of 5-15 min which are essential for the expression of the stimulating hormone effect on steroidogenesis [42,45-47]. Many attempts to identify the protein(s) have been made but none have been detected that satisfy all the functional and kinetic criteria required. Using two-dimensional gel electrophoresis a 28 kDa protein has been identified recently [48], however its functional properties are unknown. It is also not known if the active protein is synthesised de novo or is derived from a precursor protein by covalent modification [49]. 

It may be that the short half life protein has been missed because of the electrophoresis methods used. For example, a 3 kDa protein has recently been detected in adrenal cells that can stimulate steroid production in isolated mitochondria [50]. 

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