Structural differences in full-length cDNAs for two classes of sporamin, the major soluble protein of sweet potato tuberous roots
Structural differences in full-length cDNAs for two classes of sporamin, the major soluble protein of sweet potato tuberous roots
复制标题
甘薯块根主要可溶性蛋白 Sporamin 两类全长 cDNA 的结构差异
作者：
S. Murakami;T. Hattori;K. Nakamura
Sporamin占甘薯块根可溶性蛋白质总量的80%，由A和B两大类多肽组成。根据序列同源性，Sporamin cDNA克隆也可分为Sporamin A和B亚家族，亚家族内同源性远远高于亚家族间同源性。比较了孢子素B和孢子素A的一个基本全长的cDNA序列，两者的编码序列有83%的序列同源性。5‘非编码区和3’非编码区的序列除了碱基替换外，还有许多缺失。大于4bp的缺失的终点与另一个序列中存在的短直接重复的终点精确匹配，这表明这些缺失是由于DNA复制过程中错配而产生的。在孢子素B基因的5‘和3’非编码区，有5个碱基的直接重复序列，序列互补。由于这些重复序列大多不存在于SPA基因中，这些结构特征可能导致A和B两个mRNAs的二级结构不同，从而影响mRNAs的翻译效率或稳定性。这两类孢子胺的前体都带有N末端的额外序列，可分为推测的信号肽片段和富含碱性氨基酸的片段。提出了孢子胺成熟的两步法加工机理。
SummarySporamin, which accounts for 80% of the total soluble proteins in sweet potato tuberous roots, consists of two polypeptide classes, A and B. The sporamin cDNA clones can also be classified into sporamin A and B subfamilies based on their sequence homologies, with intra-subfamily homologies being much higher than inter-subfamily homologies. The sequence of an essentially full-length cDNA for sporamin B was compared with that for sporamin A. The coding sequences of two cDNAs share 83% sequence homology. The sequences in the 5′- and 3′-noncoding regions show many deletions in addition to base substitutions. The endpoints of deletions longer than 4 bp match precisely to the endpoints of short direct repeats present in the other sequence, which suggests that these deletions are generated by slipped mispairing during DNA replication. In the 5′- and 3′-noncoding region of sporamin B cDNA, there are 5 bp direct repeats with sequences complementary to each other. Since most of these repeats are absent in sporamin A cDNA, these structural features may cause a difference in the secondary structure between A and B mRNAs and affect the translational efficiencies or stabilities of the mRNAs. Precursors for both classes of sporamin carry N-terminal extra-sequences which can be separated into a putative signal peptide segment and a segment enriched with basic amino acids. A two-step processing mechanism for the maturation of sporamin is suggested.