The primary structure of human ribonuclease/angiogenin inhibitor (RAI) discloses a novel highly diversified protein superfamily with a common repetitive module.

Abstract

Immunological screening of a lambda gt11 library, constructed from HeLa mRNA, yielded several ribonuclease/angiogenin inhibitor (RAI) cDNA clones containing 900‐bp inserts. Northern blot analysis revealed that the length of the RAI mRNA is approximately 1.9 kb. Construction and screening of a eukaryotic cDNA expression library (HeLa) containing preferentially complete cDNA inserts led to the isolation of a full length clone. The complete nucleotide sequence was determined. The C‐terminal amino acid sequence deduced from the cDNA is identical to the peptide sequence obtained from a CNBr fragment of RAI, confirming the identity of the clone. The deduced primary structure of RAI consists of eight homologous tandem repeats with remarkable periodicity of leucine and cysteine residues. Each repeat is derived from the duplication of a leucine‐rich 28‐amino‐acid module. This prototype module is closely related to a repetitive 24‐amino‐acid motif of unclear function, previously found in proteins involved in important biological processes such as blood coagulation, embryonic development, cell morphogenesis and signal transduction. Although homologous, the RAI modules show distinct differences in length and amino acid composition to the modules of this group of proteins, demonstrating their high potential of variability, necessary for adaptation to very diverse roles. Based on our results we propose that these repetitive modules are a common structural feature of a novel protein superfamily whose members exert their function by highly specific protein‐protein interactions.

Specific neurotrophin binding to leucine‐rich motif peptides of TrkA and TrkB

“…LRMs are short amino acid sequences of 22 to 30 residues that are tandemly repeated in an individual protein and contain hydrophobic residues at conserved positions [27,29]. Repeats of LRMs have been known as potent mediators of strong and specific homo-or heterophilic protein-protein interactions.…”

A leucine‐rich repeat peptide derived from the Drosophila Toll receptor forms extended filaments with a β‐sheet structure

“…The C-terminal, cytoplasmic portion of the protein is homologous to that of the interleukin 1 receptor [3,4] while the extracellular domain is related to a family of membrane, extracellular matrix and cytoplasmic proteins distinguished by the presence of leucine-rich repeat sequences (LRRs). To date this family consists of yeast adenylate cyclase [S], the Drosophila adhesjon molecule chaoptin [G], mammalian proteoglycans PGi nnd PGll [7], the human lutropin-gonadotropin receptor [$I, human ribonuclease/angiogenin inhibitor [9] and the human platelet receptor glycoprotein lb (a@) complex (Gplb) [lO,l 11, Toll protein has additional sequences in common with Gplb which may constitute ligand binding sites [12]. LRRs are 22-28 amino acids long and tend to occur within proteins in tandemly repeated blocks of between 1 and 26 copies.…”

Specific binding of angiogenin to calf pulmonary artery endothelial cells.

“…However, ANG exhibits specific ribonucleolytic activity toward ribosomal and transfer RNAs (9)(10)(11)(12). A possible physiological relevance of this enzymic activity is suggested by the fact that human placental RNase inhibitor (PRI) (13)(14)(15) behaves as a potent antagonist of both the angiogenic and the ribonucleolytic activities of ANG (16,17).…”