Structural relationship of kappa-type light chains with AL amyloidosis: multiple deletions found in a VkappaIV protein

Abstract

Two amyloidogenic Bence Jones proteins (Am37 VkappaIV and NIG1 VkappaI) and one non-amyloidogenic protein (NIG26 VkappaIII) were characterized. The protein Am37 had four deletions when compared with the translated germ-line gene sequence: two Ser residues following position 27 (27e, 27f) in CDR1 and two amino acids Pro-44, and Tyr-49 in FR2 were deleted. A strictly conserved salt-bridge-forming amino acid, Asp-82, was replaced by the hydrophobic residue Leu. In a comparative study of amyloidogenic and non-amyloidogenic proteins, five amino acids (Ser-10, Ala-13, Ser-65, Gln-90, and Ile-106) were found to be unique to NIG1 and several other amyloidogenic proteins. Additional substitutions also occur within these proteins. These substitutions might be significant in altering protein folding as well as in contributing to their aggregation as amyloid fibrils.

Fig. 1

Amino acid sequences of the variable regions of the monoclonal κIV proteins. The translated germ-line (GL) gene sequence, the non-amyloidogenic and non-nephrotoxic protein LEN, the non-amyloidogenic protein FRA, the amyloidogenic protein SMA, and REC are shown relative to the amyloidogenic protein Am37 sequence. Dots indicate an amino acid residue identical to that in the GL, and solid boxes indicate deletion of an amino acid from that position. Stars represent common amino acids in all sequences. Solid circles represent novel substitutions, and thick bars indicate the β-sheet region for those positions. Small letters for residues 100–103 of SMA are not unambiguously determined. The numbering system follows Kabat et al. [4], and the β-sheet locations are superimposed from the three-dimensional structure of REI [14].

Fig. 2

Comparison of the V region sequences of VκI proteins. The V region sequence of the amyloidogenic protein NIG1 is aligned with the sequences of three amyloidogenic (A) proteins AND, ARN, and BAN, and one non-amyloidogenic (NA) protein KA. The unique residues are boxed. The numbering and other symbols are as in Fig. 1. The β-sheet locations are superimposed from the three-dimensional structure of REI [14] protein.

Fig. 3

Amino acid sequence of the variable regions of NIG26. The solid circles on single-letter codes of amino acids represent novel substitutions in the sequence.