Unusual domain pairing in a mutant of bovine lens gammaB-crystallin.
beta gamma-Crystallins from the eye lens are proteins consisting of two domains joined by a short linker. All 3D structures solved so far reveal a similar pseudo-2-fold pairing of the domains, reflecting their presumed ancient origin from a single-domain homodimer. Here we report the 2.2 A X-ray structure of the N-terminal domain of gammaB-crystallin, bearing a mutation of a residue involved in domain contacts in the native molecule (Phe56Ala). It forms a crystallographic homodimer, yet the domain orientation is different from native beta gamma-crystallins. It is considered that the new orientation derives from two structural features. (1) The replacement of the bulky phenylalanine 56 by an alanine results in a different optimal hydrophobic packing of interface residues between identical domains. (2) The paired domains have extensions derived from the domain linker, each containing a proline conserved in gamma-crystallins, and the resulting steric constraints preclude a native-like pairing but support the new arrangement. These data highlight the pivotal role of interface residues and sequence extensions in overall domain assembly.
Palme
S
S
Department of Crystallography, Birkbeck College, Malet Street, London, WC1E 7HX, UK.
Jaenicke
R
R
Slingsby
C
C
D000818:Animals; D002417:Cattle; D003459:Crystallins / Q000737:chemistry*; D018360:Crystallography, X-Ray; D008958:Models, Molecular; D009154:Mutation; D011487:Protein Conformation; D038222:gamma-Crystallins
D003459:Crystallins; C107738:crystallin, gammaB; D038222:gamma-Crystallins