Sequence and structure of VH domain from naturally occurring camel heavy chain immunoglobulins lacking light chains

Abstract

We cloned 17 different PCR fragments encoding V_H genes of camel (Camelus dromedarius). These clones were derived from the camel heavy chain immunoglobulins lacking the light chain counterpart of normal immunoglobulins. Insight into the camel V_H sequences and structure may help the development of single domain antibodies. The most remarkable difference in the camel V_H, consistent with the absence of the V_L interaction, is the substitution of the conserved Leu45 by an Arg or Cys. Another noteworthy substitution is the Leu11 to Ser. This amino acid normally interacts with the C_H1 domain, a domain missing in the camel heavy chain immunoglobulins. The nature of these substitutions agrees with the increased solubility behavior of an isolated camel V_H domain. The V_H domains of the camels are also characterized by a long CDR3, possibly compensating for the absence of the V_L contacts with the antigen. The CDR3 lacks the salt bridge between Arg94 and Asp101. However, the frequent occurrence of additional Cys residues in both the CDR1 and CDR3 might lead to the formation of a second internal disulfide bridge, thereby stabilizing the CDR structure as in the DAW antibody. Within CDRs of the camel V_H domains we observe a broad size distribution and a different amino acid pattern compared with the mouse or human V_H. Therefore the camel hypervariable regions might adopt structures which differ substantially from the known canonical structures, thereby increasing the repertoire of the camel antigen binding sites within a V_H.