Identification and structural characterization of an unusual RING-like sequence within an extracellular biomineralization protein, AP7.
The RING or Really Interesting New Gene represents a family of eukaryotic sequences that bind Zn (II) ions and participate in intracellular processes involving protein-protein interaction. Although found in over 400 different proteins, very little is known regarding the structure-function properties of these domains because of the aggregation problems associated with RING sequences. To augment this data set, we report an unusual 36 AA C-terminal sequence of an extracellular matrix mollusk shell protein, AP7, that exhibits partial homology to the RING family. This Cys, His-containing sequence, termed AP7C, binds Zn (II) and other multivalent ions, but does not utilize a tetracoordinate complexation scheme for binding such as that found in Zn (II) finger polypeptides. Moreover, unlike Zn (II) finger and RING domains, this 36 AA can fold into a relatively stable structure in the absence of Zn (II). This folded structure consists of three short helical segments (A, B, and C), with segments A and B separated by a 4 AA type I beta-turn region and segments B and C separated by a 7 AA loop-like region. Interestingly, the putative RING-like region, -RRPFHECALCYSI-, experiences slow conformational exchange between two structural states in solution, most likely in response to imido ring interconversion at P8 and P21. Poisson-Boltzmann solvation calculations reveal that the AP7C molecular surface possesses a cationic region near its N-terminus, which lies adjacent to the 30 AA mineral modification domain in the AP7 protein. Given that the AP7C sequence does not influence mineralization, it is probable that this cationic pseudo-RING region is utilized by the AP7 protein for other tasks such as protein-protein interaction within the mollusk shell matrix.
Collino
Sebastiano
S
Laboratory for Chemical Physics, Center for Biomolecular Materials Spectroscopy, New York University, 345 E. 24th Street, Room 1007, New York, New York 10010, USA.
Kim
Il Won
IW
Evans
John Spencer
JS
D000595:Amino Acid Sequence; D000818:Animals; D002113:Calcification, Physiologic / Q000502:physiology*; D002942:Circular Dichroism; D008958:Models, Molecular; D008969:Molecular Sequence Data; D008974:Mollusca; D019906:Nuclear Magnetic Resonance, Biomolecular; D011487:Protein Conformation; D011506:Proteins / Q000737:chemistry*; D017386:Sequence Homology, Amino Acid; D015032:Zinc / Q000378:metabolism; D016335:Zinc Fingers
D011506:Proteins; D015032:Zinc