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O-glycosylation is one of the most abundant and diverse types of post-translational modifications of proteins. O-glycans modulate the structure, stability, and function of proteins and serve generalized as well as highly specific roles in most biological processes. This ShapShot presents types of O-glycans found in different organisms and their principle biosynthetic pathways. To view this SnapShot, open or download the PDF.

We determined the effect on splicing of 24 point mutations in the 5' and 3' splice region of the large rabbit beta-globin intron. In vitro, 3' AG mutations drastically reduce 5' cleavage and abolish splicing. In vivo, the same mutations elicit efficient splicing at a cryptic, rather than the correct, 3' splice site. In vitro, mutations at all but 2 positions of the consensus 5' splice region impair correct splicing and promote joining of exon 1 to exon 3. In vivo, the same mutations show no effect, except for those converting 5' GT to AT or GA, which cause accumulation of lariat intermediate in vitro and in vivo. We conclude that the 5' GT need not be conserved for 5' cleavage and that it plays an important role in cleavage and exon joining at the 3' splice site.

We have generated all possible single point mutations of the invariant 5' GT of the large beta-globin intron and determined their effect on splicing in vitro. None of the mutants prevented cleavage in the 5' splice region, but many reduced or abolished exon joining. The mutations GT----TT and GT----CT resulted in a shift of the 5' cleavage site on nucleotide upstream; in the case of the mutation GT----TT, this shift was reverted by a second site mutation within the 5' splice region. Our results suggest that the 5' cleavage site is determined not by the conserved GU sequence but by the 5' splice region as a whole, most probably via base-pairing to the 5' end of the U1 snRNA.