Type I collagen is the most abundant protein in human body, produced by folding of two α1(I) and one α2(I) polypeptides into the triple helix. A conserved stem-loop structure is found in the 5' UTR of collagen mRNAs, encompassing the translation start codon. We cloned La ribonucleoprotein domain family, member 6 (LARP6) as the protein which binds the collagen 5' stem-loop in the sequence specific manner. LARP6 has a distinctive bipartite RNA binding domain, not found in other members of the La superfamily. LARP6 interacts with the two single stranded regions of 5' stemloop. The Kd for binding of LARP6 to the 5' stem-loop is 1.4 nM. LARP6 binds the 5' stem-loop in both, the nucleus and cytoplasm. In the cytoplasm, LARP6 does not associate with polysomes, however, overexpression of LARP6 blocks ribosomal loading on collagen mRNAs. Knocking down LARP6 by siRNA also decreased polysomal loading of collagen mRNAs, suggesting that it regulates translation. Collagen protein is synthesized at discrete regions of the endoplasmic reticulum (ER). We could reproduce this focal pattern of synthesis using collagen/GFP reporter protein, but only when the reporter was encoded by the mRNA with the 5' stem-loop and in the presence of LARP6. When the reporter was encoded by mRNA without the 5' stem-loop, or in absence of LARP6, it accumulated diffusely throughout the ER. This indicates that LARP6 activity is needed for focal synthesis of collagen polypeptides. We postulate that LARP6 dependent mechanism increases local concentration of collagen polypeptides for more efficient folding of the collagen heterotrimer.

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Nitric oxide (NO) functions as an intercellular messenger and mediates numerous biological functions. Among the three isoforms of NO synthase that produce NO, the ubiquitously expressed neuronal NO synthase (nNOS) is responsible for a large part of NO production, yet its regulation is poorly understood. Recent reports of two alternative spliceforms of nNOS in the mouse and in man have raised the possibility of spatial and temporal modulation of expression. This study demonstrates the existence of at least three transcripts of the rat nNOS gene designated nNOSa, nNOSb, and nNOSc, respectively, with distinct 5 Ј untranslated first exons that arise from alternative splicing to a common second exon. Expression of the alternative transcripts occurs with a high degree of tissue and developmental specificity, as demonstrated by RNase protection assays on multiple tissues from both fetal and adult rats.

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