Molecular Evolution of Ryanodine Receptor Isoforms.
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The Sarcoplasmic Reticulum Calcium ion channel (SR) functions primarily as an intracellular store of calcium in skeletal muscle cells. The SR channel is responsible for the controlled release of calcium in skeletal muscle cells during muscular contraction/relaxation and movement. Due to its high affinity for the plant alkaloid Ryanodine, the SR calcium channel is commonly referred to as the ryanodine receptor (RyR). Presently, three known RyR types have been identified: RyR1, RyR2, and RyR3. The RyR1 type is predominately expressed in skeletal muscles and the cerebellum. The RyR2 type has been observed primarily in cardiac muscle and brain tissues. The RyR3 type shows expression in a variety of tissues. A full-length message has been cloned from a blue marlin cDNA library. A comparison of its amino acid sequence to other known sequences shows this clone to match other previously described RyR isoforms, (Franck et al. , 1998). Two distinct RyR1-like messages have been cloned and characterized in fish. These isoforms cluster phylogenetically with other RyR1 isoforms but are discretely expressed in the fast and slow twitch muscle fibers. These results are indicative of a gene duplication event. Generally, these isoforms are expressed primarily in a tissue specific manner, however, a slow-twitch isoform sequence has been amplified from fast-twitch tissues for both tuna and marlin muscle. Additionally, isoform sequences have been amplified from Bluefin Tuna, Albacore Tuna, and a phylogenetically distant outgroup, Roosterfish. A region within the isoform sequences show the retention of a 72 base pair (bp) intron. The sequence of the retained intron contains an inframe stop codon which codes for the truncation of the RyR protein, rendering the channel inactive. Intron retention in the slow transcript only occurs in fast twitch muscle fiber type tissue. The retained intron thus appears to function as a means of transcriptional control, preventing the expression of slow twitch isoforms in fast-twitch fibers.