Methionine-rich repeat proteins: a family of membrane-associated proteins which contain unusual repeat regions.
We report the protein isolation, cloning and characterization of members of an unusual protein family, which comprise the most abundant proteins present in the squid eye. The proteins in this family have a range of molecular weights from 32 to 36 kDa. Electron microscopy and detergent solubilization demonstrate that these proteins are tightly associated with membrane structures where they may form tetramers. Despite this, these proteins have no stretches of hydrophobic residues that could form typical transmembrane domains. They share an unusual protein sequence rich in methionine, and contain multiple repeating motifs. We have therefore named these proteins Methionine-Rich Repeat Proteins (MRRPs). The use of structure prediction algorithms suggest very little recognized secondary structure elements. At the time of cloning no sequence or structural homologues have been found in any database. We have isolated three closely related cDNA clones from the MRRP family. Coupled in vitro transcription/translation of the MRRP clones shows that they encode proteins with molecular masses similar to components of native MRRPs. Immunoblot analysis of these proteins reveals that they are also present in squid brain, optic lobe, and heart, and also indicate that MRRP-like protein motifs may also exist in mammalian tissues. We propose that MRRPs define a family of important proteins that have an unusual mode of attachment or insertion into cell membranes and are found in evolutionarily diverse organisms.
Weiss
Jamie L
JL
Institute of Molecular Physiology, Biomedical Science, The University of Sheffield, Western Bank, Sheffield S10 2TN, UK.
Evans
Nicholas A
NA
Ahmed
Tanweer
T
Wrigley
Jonathan D J
JD
Khan
Shukria
S
Wright
Charles
C
Keen
Jeffrey N
JN
Holzenburg
Andreas
A
Findlay
John B C
JB
D000595:Amino Acid Sequence; D000818:Animals; D002462:Cell Membrane / Q000502:physiology* / Q000648:ultrastructure*; D002478:Cells, Cultured; D049832:Decapodiformes; D005123:Eye / Q000378:metabolism* / Q000648:ultrastructure*; D008565:Membrane Proteins / Q000737:chemistry / Q000378:metabolism* / Q000648:ultrastructure*; D008715:Methionine / Q000737:chemistry / Q000378:metabolism*; D008969:Molecular Sequence Data; D020449:Repetitive Sequences, Amino Acid / Q000502:physiology; D014018:Tissue Distribution
D008565:Membrane Proteins; D008715:Methionine