Multiple Translational Isoforms Give Functional Specificity to Serum- and Glucocorticoid-induced Kinase 1

Abstract

Serum-and glucocorticoid-induced kinase 1 is a ubiquitous kinase that regulates diverse processes such as ion transport and cell survival. We report that a single SGK1 mRNA produces isoforms with different N-termini owing to alternative translation initiation. The long isoforms, 49 and 47 kDa, are the most abundant, localize to the ER membrane, exhibit rapid turnover, their expression is decreased by ER stress, activate the epithelial sodium channel (ENaC) and translocate FoxO3a transcriptional factors from the nucleus to the cytoplasm. The short isoforms, 45 and 42 kDa, localize to the cytoplasm and nucleus, exhibit long half-life and phosphorylate glycogen synthase kinase-3␤. The data indicate that activation of Sgk1 in different cellular compartments is key to providing functional specificity to Sgk1 signaling pathways. We conclude that the distinct properties and functional specialization of Sgk1 given by the N-terminus confer versatility of function while maintaining the same core kinase domain.

A brain-specific SGK1 splice isoform regulates expression of ASIC1 in neurons

“…In a similar manner, removal of the N terminus of the canonical SGK1 eliminates regulation of ENaC (27). Therefore, this result confirms and extends our previous conclusion that correct subcellular localization is key for achieving functional specificity of the SGK1 isoforms (23). Regulation by compartmentation is partly overcome by rendering the cytosolic SGK1.1 mutants constitutively active because, owing to their distribution over the whole cell, they have access to substrates that cannot be reached under normal conditions.…”

“…This disparity is due to rapid degradation by the ubiquitin/proteasomal system (22). We used a transgenic mouse strain with insertion of a bacterial artificial chromosome (BAC) containing the whole-mouse SGK1 gene (Ϸ200 kb) modified by the addition of three HA epitopes at the C terminus of the coding region (23). In tissue homogenates of brain, heart, and lung, we identified SGK1 (49/45 kDa) and SGK1.1 (60 kDa) only in brain ( Fig.…”

Epithelial sodium channel regulated by differential composition of a signaling complex

“…In particular, it is interesting to recall recent studies that showed the existence of 4 independent SGK1 products that arise because of use of alternate start sites during translation (32). Interestingly, we observed that although 4 separate SGK1 variants are consistently expressed in total cellular lysates (Fig.…”

“…The effect of GILZ1 seems to be selectively on the longer SGK1 product that presumably has the N terminus intact. In this context, it is pertinent to recall earlier studies from our laboratory and others that showed that the N terminus of SGK1 is important for its ENaC-stimulatory activities via Nedd4-2 inhibition (5,32). Further studies will be needed to elucidate the role of the N terminus and PY motif of SGK1 in the context of the ENaC regulatory complex.…”

Alternative translation start sites and hidden coding potential of eukaryotic mRNAs

“…It was proposed that leaky scanning is the mechanism responsible for the synthesis of these protein variants characterized by different stabilities, subcellular localizations and functions. (36) Similarly, alternative TIS and leaky scanning are used to produce functionally different isoforms of a regulator of G-protein signaling (RGS2) and human glucocorticoid receptor (hGRalpha). (37,38) IRES can also mediate the translation of several protein isoforms with different subcellular localizations: for example, fibroblast growth factor-2 (FGF-2) can be produced as distinct isoforms by the alternative initiation of translation on an IRES-containing mRNA and these isoforms are differently sorted in cells.…”