Increased structural and combinatorial diversity in an extended family of genes encoding Vlp surface proteins of Mycoplasma hyorhinis

Abstract

Variable lipoproteins (Vlp) constitute the major coat protein of Mycoplasma hyorhinis. They are products of multiple, divergent, single-copy genes organized in a chromosomal cluster. Three genes, vlpA, vlpB, and vlpC, have been previously identified in clonal isolates of M. hyorhinis SK76. Each is linked to a characteristic promoter region containing a homopolymeric tract of adenine residues [poly(A) tract], subject to hypermutation, that transcriptionally controls phase variation of vlp genes and leads to combinatorial surface mosaics of distinct Vlp products. The size of the natural vlp gene repertoire is unknown but may critically determine the degree of structural and combinatorial diversity available in this species. In this study, the vlp repertoire of M. hyorhinis GDL-1 was characterized and shown to contain three additional genes, vlpD, vlpE, and vlpF, clustered with other known vlp genes in the order 5-vlpD-vlpE-vlpF-IS-vlpA-IS-vlpB-vlpC-3, where IS represents copies of the IS1221 element of M. hyorhinis. The 5 boundary of this expanded family was identical to that of the more limited family 5-vlpA-IS-vlpB-vlpC-3 previously described in a clonal isolate of strain SK76. A recombinant construct containing vlpD, vlpE, and vlpF expressed antigenically distinguishable products corresponding to each gene. These genes encode characteristic C-terminal repetitive regions that are subject to size variation by insertion or deletion of intragenic repeats but maintain an extended, charged structure. Each vlp gene also contained characteristic alternative open reading frames, which provide a potential reservoir of coding sequence for Vlp diversity, possibly recruited through insertion and/or deletion mutations. These findings demonstrate a vastly expanded potential for structural diversity and combinatorial display of surface mosaics on this organism and suggest that modulation of the vlp repertoire, possibly in conjunction with mobile elements, may determine the capacity for surface variation in natural populations and laboratory strains of this mycoplasma species.The variable lipoprotein (Vlp) system of Mycoplasma hyorhinis provides a mutational strategy for adaptive surface variation that generates extreme population diversity in the major coat proteins expressed on this wall-less procaryote (27,28,38,40). The complexity of this system and its role as a prototype for analogous emerging systems in other mycoplasma species warrants a detailed characterization of the genetic and structural attributes contributing to its adaptive capabilities. Vlps are procaryotic lipoproteins processed by cleavage of a conserved prolipoprotein sequence (11) and expressed as abundant surface proteins anchored on the single mycoplasma membrane by a lipid moiety on an N-terminal Cys residue. Molecular genetic analysis has shown that alternative Vlps share distinctive structural features despite divergence throughout their surface-exposed protein sequence. Structural details of known Vlps (VlpA, VlpB, and VlpC) have been defined at the g...

Cloning and functional analysis of the P97 swine cilium adhesin gene of Mycoplasma hyopneumoniae

“…There was an apparent 17-amino-acid membrane-spanning hydrophobic region near the amino terminus (data not shown). There was no identifiable lipoprotein acylation site, such as has been observed in other mycoplasmal membrane proteins (29,33), however. Structural analysis of the predicted protein sequence indicated a strong ␣-helical nature throughout the protein when the Robson-Garnier algorithm (8) was used but substantially less when the method of Chou and Fasman (2) was used (data not shown), in contrast to the results of King et al (13).…”

The Mycoplasma hominis P120 membrane protein contains a 216 amino acid hypervariable domain that is recognized by the human humoral immune response

“…The repetitive elements are suggested to be of importance for antigenic variation (Peterson et al, 1995). In the size-variable Vlps in Mycoplasma byorbinis and V-1 in Mycoplasma pulmonis, repetitive elements play a major role in generating antigenic variation (Simmons et al, 1996 ;Yogev et al, 1995). However, exchange of larger cassettes of DNA does not seem to be important for antigenic variation in these genes.…”

Gene Families Encoding Phase- and Size-Variable Surface Lipoproteins of Mycoplasma hyorhinis

“…Defining new or consistent features of the Vlp system, including additional vlp genes, their comparative organization in families, and their common structural features, is an essential step in predicting the functions of the Vlp system in adaptive variation. Comparison of cloned isolates of M. hyorhinis strains SK76 and GDL showed that the vlp repertoires in these two strains comprised three and six vlp genes, respectively (21,22). However, evidence raising the possibility of additional vlp genes in other isolates of strain SK76 has been reported (22).…”

“…In addition to its natural host, M. hyorhinis occupies an important artificial niche as one of the most prevalent mycoplasmal tissue culture contaminants worldwide (7). Although the origins and initial pathway for transmitting these cell culture-adapted contaminants are not known, one such isolate, type strain GDL (1), has been characterized and shown to contain additional genes of the Vlp family (10,22).…”