Coordination of immunoglobulin chain folding and immunoglobulin chain assembly is essential for the formation of functional IgG

BiP and immunoglobulin light chain cooperate to control the folding of heavy chain and ensure the fidelity of immunoglobulin assembly

The immunoglobulin (Ig) molecule is composed of two identical heavy chains and two identical light chains (H2L2). Transport of this heteromeric complex is dependent on the correct assembly of the component parts, which is controlled, in part, by the association of incompletely assembled Ig heavy chains with the endoplasmic reticulum (ER) chaperone, BiP. Although other heavy chain-constant domains interact transiently with BiP, in the absence of light chain synthesis, BiP binds stably to the first constant domain (CH1) of the heavy chain, causing it to be retained in the ER. Using a simplified two-domain Ig heavy chain (VH-CH1), we have determined why BiP remains bound to free heavy chains and how light chains facilitate their transport. We found that in the absence of light chain expression, the CH1 domain neither folds nor forms its intradomain disulfide bond and therefore remains a substrate for BiP. In vivo, light chains are required to facilitate both the folding of the CH1 domain and the release of BiP. In contrast, the addition of ATP to isolated BiP-heavy chain complexes in vitro causes the release of BiP and allows the CH1 domain to fold in the absence of light chains. Therefore, light chains are not intrinsically essential for CH1 domain folding, but play a critical role in removing BiP from the CH1 domain, thereby allowing it to fold and Ig assembly to proceed. These data suggest that the assembly of multimeric protein complexes in the ER is not strictly dependent on the proper folding of individual subunits; rather, assembly can drive the complete folding of protein subunits.

The in vivo association of BiP with newly synthesized proteins is dependent on the rate and stability of folding and not simply on the presence of sequences that can bind to BiP

Immunoglobulin heavy chain-binding protein (BiP) is a member of the hsp70 family of chaperones and one of the most abundant proteins in the ER lumen. It is known to interact transiently with many nascent proteins as they enter the ER and more stably with protein subunits produced in stoichiometric excess or with mutant proteins. However, there also exists a large number of secretory pathway proteins that do not apparently interact with BiP. To begin to understand what controls the likelihood that a nascent protein entering the ER will associate with BiP, we have examined the in vivo folding of a murine lambdaI immunoglobulin (Ig) light chain (LC). This LC is composed of two Ig domains that can fold independent of the other and that each possess multiple potential BiP-binding sequences. To detect BiP binding to the LC during folding, we used BiP ATPase mutants, which bind irreversibly to proteins, as "kinetic traps." Although both the wild-type and mutant BiP clearly associated with the unoxidized variable region domain, we were unable to detect binding of either BiP protein to the constant region domain. A combination of in vivo and in vitro folding studies revealed that the constant domain folds rapidly and stably even in the absence of an intradomain disulfide bond. Thus, the simple presence of a BiP-binding site on a nascent chain does not ensure that BiP will bind and play a role in its folding. Instead, it appears that the rate and stability of protein folding determines whether or not a particular site is recognized, with BiP preferentially binding to proteins that fold slowly or somewhat unstably.

Assembly, sorting, and exit of oligomeric proteins from the endoplasmic reticulum

The endoplasmic reticulum (ER) uses various mechanisms to ensure that only properly folded proteins enter the secretory pathway. For proteins that oligomerize in the ER, the proper tertiary and quaternary structures must be achieved before their release. Although some proteins fold before oligomerization, others initiate oligomerization cotranslationally. Here, we discuss these different strategies and some of the unique problems they present for the ER quality control system. One mechanism used by the ER is thiol retention. Thiol retention operates by monitoring the redox state of specific cysteine residue(s) and was discovered in studies on the assembly of IgM, a complex oligomeric glycoprotein. This system is also involved in retaining other unassembled proteins in the ER. Mutations that result in uneven numbers of cysteine residues can subject yet other proteins to thiol retention, altering their oligomerization status and function. The implications of these results on the effects of thiol retention on protein function and cell fate are discussed.

Inefficient Assembly and Intracellular Accumulation of Antibodies with Mutations in VH CDR2

We previously described secretion defects in four mutants of the murine anti-phosphocholine Ab, T15. The mutant heavy (H) chains had amino acid replacements in the VH complementarity-determining region 2 (HCDR2) and were expressed at normal intracellular levels. Here, the intracellular fate of the secretion-defective mutant heavy chains was investigated. Metabolic labeling demonstrated that the T15 wild-type Ab was secreted within a 4-h chase. In contrast, the mutant H chains accumulated with intracellular t1/2 values ranging from 10 to 24 h. The mutant H chains were associated with increased levels of the molecular chaperones BiP and GRP94, and remained endoglycosidase H sensitive, suggesting retention in the endoplasmic reticulum. Assembly of the mutant H chains with T15 light (L) chain was arrested at the H2 and H2L intermediate stages of the T15 wild-type pathway (H2 → H2L → H2L2). Even though some assembly with L chain occurred, it was not as a secretion-competent H2L2 Ig moiety. The T15 L chains coexpressed with mutant H chains were degraded efficiently except for a minor L chain population with a long t1/2 that was apparently protected at the H2L stage. To our knowledge, this is the first study demonstrating that intracellular half-lives of Ig H and L chains can be influenced by somatic mutations in HCDR2.

Intracellular antibodies (intrabodies) for gene therapy of infectious diseases

Intracellular antibodies (intrabodies) represent a new class of neutralizing molecules with a potential use in gene therapy. Intrabodies are engineered single-chain antibodies in which the variable domain of the heavy chain is joined to the variable domain of the light chain through a peptide linker, preserving the affinity of the parent antibody. Intrabodies are expressed inside cells and directed to different subcellular compartments where they can exert their function more effectively. The effects of intrabodies have been investigated using structural, regulatory, and enzymatic proteins of the human immunodeficiency virus (HIV-1) as targets. These intrabodies have demonstrated their versatility by controlling early as well as late events of the viral life cycle. In this article, we review studies of the use of intrabodies as research tools and therapeutic agents against HIV-1.

Recombinant antibodies with conformationally constrained HIV type 1 epitope inserts elicit glycoprotein 160-specific antibody responses in vivo

Although neutralizing epitopes have been identified on the HIV-1 gp120/gp41 envelope complex, efforts to exploit this information through the construction of synthetic peptide vaccines have been largely unsuccessful. Unfortunately, synthetic peptides tend to be poorly immunogenic, and most often lack the conformational characteristics of the corresponding epitope in the native protein. In an effort to circumvent these difficulties, we have utilized an anti-class II MHC antibody as a molecular scaffold for the construction of two conformationally constrained neutralizing HIV-1 epitopes. Previously we demonstrated that anti-class II MHC antibodies can function as vectors for the induction of adjuvant-independent antibody responses to incorporated epitopes. In this instance, one epitope, IHIGPGRAFYT, is the crown of the V3 loop from gp120, and the other, ELDKWAS, is a neutralizing epitope from gp41. The insertion of these epitopes into a specific loop region of the immunoglobulin heavy chain FR3 was found to preserve the anti-class II MHC-binding activity of these recombinant antibodies, and the inserts were recognized by epitope specific monoclonal antibodies. When utilized as immunogens, each of these epitope insertion antibodies was able to induce high-titer anti-HIV-1 gp160 responses in guinea pigs. These responses were conformation specific in that the anti-gp160 binding was not inhibited by the synthetic peptide corresponding to the epitope in question. These data demonstrate the potential to construct conformationally constrained HIV-1 epitope immunogens, and thus establish an alternative approach to the design of an effective HIV-1 subunit vaccine.

Assembly of immunoglobulin light chains as a prerequisite for secretion. A model for oligomerization-dependent subunit folding

Oligomeric proteins usually have to assemble into their final quartenary structure to be secreted. However, most immunoglobulin (Ig) light (L) chains can be exported as free chains, whereas only a few Ig L chains, here referred to as export-incompetent, have to assemble with Ig heavy (H) chains into antibody molecules to be secreted. In the absence of Ig H chain expression, these export-incompetent Ig L chains remain bound to BiP as partially folded monomers with only one of the two internal disulfide bonds being formed. To understand the apparent discrepancy in Ig L chain export, we performed assembly studies with chimeric Ig chains and found that the variable (V) domain of the export-incompetent NS1 kappa chain cannot mediate homodimer formation. Conversely, the V domain of the export-competent J558L lambda1 chain supports homodimer formation and, concordantly, these Ig L chains are secreted as noncovalently or covalently linked homodimers. We show that the export-incompetent mutant lambda1 FS62 chain forms disulfide bonds in both domains only upon pairing with Ig H chain and is secreted as part of an antibody. Therefore, Ig L chain assembly seems to be a prerequisite for complete folding, indicating that Ig L chain secretion generally depends on either homo- or heterodimer formation. We discuss a mechanism that controls oligomerization by monitoring the conformation of individual subunits that cannot proceed in folding prior to successful assembly.

Composition and function of T-cell receptor and B-cell receptor complexes on precursor lymphocytes

The precursor T-cell receptors (TCRs) and B-cell receptors (BCRs) direct lymphocyte development to the mature T-cell and B-cell stage, respectively. Recent genetic and biochemical experiments reveal the striking parallel in structure and function of these receptors. They consist of TCR beta and BCR mu chains paired with surrogate TCR alpha and BCR light chains. Both receptors employ a two-component signal transduction unit: CD3 gamma epsilon for the pre-TCR, and CD79ab for the pre-BCR. Plasma membrane levels of pre-TCR/BCR complexes are kept extremely low, most probably by a mechanism involving specific retention in the endoplasmic reticulum. This mechanism may control the signalling activity of pre-TCR/BCR and therewith the lymphocyte differentiation process.

BiP binding sequences in antibodies

During the process of folding and assembly of antibody molecules in the endoplasmic reticulum, immunoglobulin heavy and light chains associate transiently with BiP, a resident endoplasmic reticulum protein that is a member of the Hsp70 family of molecular chaperones. BiP is thought to recognize unfolded or unassembled polypeptides by binding extended sequences of approximately seven amino acids that include bulky hydrophobic residues not normally exposed on the surface of native proteins. We used a computer algorithm developed to predict BiP binding sites within protein primary sequences to identify sites within immunoglobulin chains that might mediate their association with BiP. Very few of the sequential heptapeptides in the heavy or light chain sequences were potential BiP binding sites. Analysis of the ability of synthetic heptapeptides corresponding to 24 potential sites in heavy chains to stimulate the ATPase activity of BiP indicated that at least half of them were authentic BiP binding sequences. These sequences were not confined to a single domain of the heavy chain but were distributed within both the VH and CH domains. Interestingly, when the BiP binding sequences were mapped onto the three-dimensional structure of the Fd antibody fragment, the majority involve residues that participate in contact sites between the heavy and light chains. Therefore, we suggest that in vivo BiP chaperones the folding and assembly of antibody molecules by binding to hydrophobic surface regions on the isolated immunoglobulin chains that subsequently participate in interchain contacts.