Identification of N-terminal methionine in the precursor of immunoglobulin light chain. Initiation of translation of messenger ribonucleic acid in plants and animals

An experimental approach to enumerate the genes coding for immunoglobulin variable-regions

Critical to our understanding of the immune system diversity is the determination of the number of germ line V genes. The total number of V genes is given by the product: number of subgroups x number of germ line genes per subgroup. Studies of kappa chains and of embryonic DNA indicate 5-10 V genes per subgroup. Statistical analysis of the limited sequence data of mouse kappa chains suggest about 50 V kappa subgroups. We report here a general approach for direct estimation of the number of VL and VH subgroups expressed in normal spleen, and present data for V kappa. The kappa mRNA of the spleen is a heterogeneous population where different V kappa are linked to the same C kappa, i.e. C kappa equals total V kappa. The ratio C kappa/distinct V kappa approximates the number of subgroups since V kappa of the same subgroup cross hybridize while V kappa of different subgroups do not. This ratio was determined by molecular hybridization of cloned C kappa and V kappa DNA probes with spleen mRNA. The results indicate the expression of 280 V kappa subgroups in mouse. Assuming an average of 7 genes per subgroup, we estimate about 2000 V kappa germ line genes.

Synthesis of part of a mouse immunoglobulin light chain in a bacterial clone

We have cloned double stranded cDNA sequences encoding a mouse immunoglobulin light chain (L-321) into the PstI site of the beta-lactamase gene of plasmid pBR322 by the oligo (dG)-oligo (dC) tailing procedure. Escherichia coli X1776 transformed by the recombinant plasmids were screened for the expression of L-321 antigenic determinants by a newly developed in situ radio-immunoassay. One out of seven transformants screened was found to synthesize an L-chain like protein. Each bacterial cell produces about 550 molecules of the L-chain sequence. Preferential segregation of the L-chain sequence to the periplasmic space suggest covalent attachment of the L-chain sequence to the N-terminal portion of beta-lactamase. Restriction mapping of the plasmid DNA isolated from the positive clone indicated the presence of a DNA sequence coding for the entire constant region and extending into the variable region for a length corresponding to about 40 amino acid residues. The orientation of the cloned cDNA with respect to the plasmid DNA is compatible with the formation of a fused beta-lactamase-L-321 peptide.

Primary structure of the NH2-terminal extra piece of the precursor to human placental lactogen

The cell-free translation product of human placental lactogen mRNA is a precursor molecule larger than the mature hormone that circulates in plasma. To determine the structure of pre-placental lactogen, the poly(A)-rich RNA fraction of term placenta was isolated and translated in a wheat germ cell-free system. The mRNA programmed the synthesis of a major protein, 3000 daltons larger than placental lactogen, that was specifically precipitated by hormone antibodies. The immunoprecipitated protein was labeled separately with 20 radioactive amino acids and subjected to sequence analysis. The results showed the synthesis of pre-placental lactogen in which an extra piece 25 residues long preceded the NH2 terminus of the mature protein. The structure of the extra piece is as follows: Met-Pro-Gly-Ser-Arg-Thr-Ser-Leu-Leu-Ala-Phe-Ala-Leu-Leu-Cys-Leu-Pro-Trp-Leu-Gln-Glu-Ala-Gly-Ala-. Met1 is the initiator residue because only initiator [35S]Met-tRNAMet1, but not internal [35S]Met-tRNA2Met, donated NH2-terminal methionine. The structure of the extra piece showed little homology with that of unrelated hormones but striking homology (64%) with the extra piece of rat pre-growth hormone. Most amino acid substitutions involved a single base change in the codon. Mature human placental lactogen and rat growth hormone have 59% homology in sequence. Thus, our findings provide additional evidence to support the common evolutionary origin of these hormones, not only of the mature proteins but also of the extra piece segments.

Messenger RNA of opsin from bovine retina: isolation and partial sequence of the in vitro translation product

Opsin, the apoprotein of the visual pigment rhodopsin, is synthesized on membranes of the rough endoplasmic reticulum and subsequently passes through the Golgi apparatus to the rod outer segment. This pathway parallels the early stages of biosynthesis of some secretory proteins and viral membrane glycoproteins. Most of these proteins are initially synthesized as precursor molecules with a short-lived hydrophobic extra peptide segment at the NH(2) terminus. Therefore we investigated whether or not the immediate translation product of opsin mRNA contains a similar short-lived NH(2)-terminal extra peptide. The mRNA coding for opsin was isolated from bovine retina polysomes precipitated by antibodies to opsin. The mRNA directed the cell-free synthesis of a protein comparable in size to opsin that was specifically precipitated by anti-opsin antibodies. Sequence analyses of the immunoprecipitated protein labeled with six radioactive amino acids (Met, Asn, Pro, Phe, Tyr, Val) provided the following result: [Formula: see text] (X is unknown). This partial sequence of the cell-free product corresponds exactly to the published NH(2)-terminal segment of native opsin (21 residues long) and extends beyond this region. Met-1 was shown to be the initiator methionine residue, because only the initiator [(35)S]Met-tRNA(1) (Met)-not the internal [(35)S]Met-tRNA(2) (Met)-donated the NH(2)-terminal methionine. This finding essentially rules out the possibility that Met-1 was preceded by a peptide that was rapidly cleaved. Thus opsin, and not a precursor, is the immediate product of opsin mRNA translation.

B lymphocytes contain three species of mu chains

The murine B cell line cloned from a single cell, 38C-13, synthesizes three species of mu chains, that of cell surface membrane IgM (m-mu), that of secreted IgM (s-mu) and that of intracellular IgM (i-mu). They differ in their mobility on sodium dodecyl sulfate polyacrylamide gel electrophoresis. Sequence analysis of the different mu chains suggests that they are identical in the N-terminal as well as in their C-terminal positions. The ratio between incorporated radioactive monosaccharides to radioactive amino acids into the three different mu chains was higher in s-mu than in m-mu, but nevertheless m-mu migrated more slowly than s-mu on sodium dodecyl sulfate polyacrylamide gel electrophoresis. However, since this ratio may also be influenced by the rate of synthesis, it may not represent a real molar ratio of carbohydrate to protein. Studies with normal spleen cells clearly indicated the presence of the same three types of mu chains.

On a regulatory gene controlling the expression of the murine lambda1 light chain

We describe here two alleles, an allele of the lambda1 locus present in the SJL strain (rlambda1lo) and an allele of the lambda1 locus present in the BALB/c strain (rlambda1 +), of a regulatory gene locus which specifically influences the expression of the mouse lambda1 light chain structural gene. The rlambda1 regulatory gene is not linked to either the major histocompatibility complex or to the heavy-chain allogroup but appears to be linked to the lambda1 structural gene locus. In the homozygous state, the present of the rlambda1lo allele results in a 50-fold reduction in the number of lambda1 antigen-sensitive, bone-marrow derived lymphocytes (ASCs) compared to the presence of the rlambda1 + allele. However, those few lambda1ASCs present in rlambda1lo homozygotes can be induced normally to produce lambda1 light chains indistinguishable from those found in rlambda1 + homozygotes. The reduction in lambda1ASC's due to the rlambda1lo allele results both in a reduction in the amount of lambda1 Ig in the serum and also in a large variation in the magnitude of the lambda1 antibody response to alpha(1,3) dextran by individual animals. This variation permits the estimate that, on the average, 50 B cells of anti-alpha(1,3) specificity must be present per animal to permit a measurable response. Surprisingly, the expression of a gene locus regulating lambda1 light chain expression (rlambda1 locus) shows a clear gene dosage effect with rlambda1lo/rlambda1 + heterozygotes having 1/2 the number of lambda1ASCs and 1/2 the amount of serum lambda1 Ig as rlambda1 +/rlambda1 + homozygotes. This fact permits an analysis of the relationship between germ-line v-genes and their individual expression in serum Ig. The rlambda1 locus controls specifically a DNA-level event which occurs in stem cells as they become committed to lambda1 light chain expression. We postulate that the rlambda1 locus represents one of the DNA level recognition sites involved in the translocation event which places the vlambda1 and clambda1 structural genes in a transcriptional unit.

Amino acid sequence of the precursor region of MOPC-315 mouse immunoglobulin heavy chain

Partially purified mRNA coding for the MOPC-315 heavy immunoglobulin alpha chain was translated in a reticulocyte lysate containing 20 labeled amino acids. Radiolabeled MOPC-315 heavy chain precursor protein, purified by preparative gel electrophoresis and immunoprecipitation, was sequenced by Edman degradation. The labeled phenylthiohydantoin amino acid obtained in each cycle was identified and quantitated by high-pressure liquid chromatography. The precursor sequence of 18 amino acids, Met-Lys-Val-Leu-Ser-Leu-Leu-Tyr-Leu-Leu-Thr-Ala-Ile-Pro-His-Ile-Met-Ser, preceded the sequence corresponding to the NH2 terminus of the mature secreted heavy chain.

Independent expression of the gene coding for the constant domain of immunoglobulin light chain: evidence from sequence analyses of the precursor of the constant region polypeptide

The mRNA coding for the kappa-type constant region (C(kappa)) was purified from two clones derived from the MPC-11 mouse myeloma. This mRNA directs the cell-free synthesis of a C(kappa) precursor (molecular weight, about 15,000) in which an extra piece, 17 residues long, precedes the NH(2)-terminal residue (Ala(109)) of the C(kappa) region. The partial sequence of the extra piece is: Met-X-Thr-Asp-Thr-Leu-Leu-Leu-Trp-Val-Leu-Leu-Leu-Trp-Val-Pro-X- (X is unknown). Met(1) was shown to be the initiator methionine. The sequence of the C(kappa) extra piece is completely different from any known sequence preceding residue Ala(109) in whole light (L) chains, thus establishing that the C(kappa)-region mRNA could not have originated from mRNA coding for the whole L chain. The structural features of the C(kappa) extra piece (marked hydrophobicity, size, and a methionine at the NH(2)-terminus) are identical to those characteristic of the NH(2)-terminal extra piece linked to the variable (V) region of whole L-chain precursors. In addition, the C(kappa) extra piece and the extra piece linked to the V region of MOPC-321 L chain have 70% sequence homology. These findings can be explained by the two genes-one Ig chain hypothesis, if we assume that the DNA coding for the extra piece (xp-DNA) is a constitutive part of the V gene. According to this model, the C(kappa)-region mRNA could have originated from: (i) translocation of this V gene to the C gene, deletion of the entire mature V gene, and "end-to-end" repair of the remaining xp-DNA to the C gene; (ii) translocation to the C gene only of the xp-DNA portion of the V gene. Alternatively, we may assume that the xp-DNA is not covalently linked to the mature V gene at all times, as might be the case for the DNA of hypervariable regions presumed to be in episomes. This raises the intriguing speculation that the xp-DNA represents a third distinct gene, designated xp-gene. The presumed xp-gene may be involved in the regulation of gene transcription: when linked to the mature V gene it initiates a chain of events leading to whole L-chain mRNA formation; when attached to the C gene it leads to its transcription to provide the C-region mRNA.

Glutamine as a precursor to N-terminal pyrrolid-2-one-5-carboxylic acid in mouse immunoglobulin lambda-type light chains. Amino acid-sequence variability at the N-terminal extra piece of lambda-type light-chain precursors

The mRNA molecules coding for three mouse immunoglobulin lambda-type light (L) chains (MOPC-104E lambda(1), RPC-20 lambda(1), MOPC-315 lambda(2)) programme the cell-free synthesis of precursors larger than the mature proteins. Radioactive amino acid-sequence analyses of each of the three precursors labelled with [(3)H]alanine, [(3)H]serine, [(3)H]glutamine, [(3)H]glutamic acid and [(3)H]threonine showed that an extra piece, at least 18 residues long, is linked to the N-terminus of the mature L-chains. The N-terminal extra-peptide segment may be 19 residues long, since analyses of precursors labelled with [(35)S]methionine indicated an additional N-terminal methionine residue which was recovered in low yields. Presumably this is the initiator methionine, which is known to be short lived in eukaryotes. The mature forms of MOPC-104E, RPC-20 and MOPC-315 lambda L-chains are blocked at the N-termini by pyrrolid-2-one-5-carboxylic acid (pyroglutamic acid). Sequence analyses of precursors labelled with [(3)H]glutamine and [(3)H]glutamic acid showed incorporation only of glutamine in a position that matches with the position of pyrrolid-2-one-5-carboxylic acid in the mature forms of all three precursors, and incorporation of glutamic acid in other positions. The data showed the absence of glutamine-glutamic acid interconversion, since the radioactive peaks obtained from either (3)H-labelled amino acid were discrete, and free from cross-contamination. These results prove that glutamine is the precursor amino acid of pyrrolid-2-one-5-carboxylic acid at the N-termini of the mature MOPC-104E lambda(1), RPC-20 lambda(1) and MOPC-315 lambda(2) L-chains. Thus the formation of pyrrolid-2-one-5-carboxylic acid by cyclization of glutamine is a post-translational event which occurs after, or concomitant with, cleavage of the extra piece from the precursor to yield the mature L-chain. The variable (V) regions (110 amino acid residues) of mouse lambda L-chains are quite similar: when compared with that of MOPC-104E lambda(1) chain, the V-region of RPC-20 lambda(1) chain differs in one residue, and the V-region of MOPC-315 lambda(2) chain differs in 11 residues. The partial sequence data show that the N-terminal extra pieces of the two lambda(1) L-chain precursors have, so far, identical partial sequences; the extra piece of the lambda(2) L-chain precursor differs from these in at least three out of 19 positions.

Amino acid sequence of the NH2-terminal extra piece segments of the precursors of mouse immunoglobulin lambda1-type and kappa-type light chains

The mRNA molecules coding for mouse immunoglobulin light (L) chains direct the cell-free synthesis of precursors in which extra peptide segments precede the amino termini of the mature proteins. The results of amino acid sequence analyses of two precursors labeled with 20 radioactive amino acids enabled unambiguous determination of the complete primary structure of the extra piece segments. The complete sequences (and sizes) of the NH2-terminal extra pieces are: in MOPC-104E lambda1 L-chain precursor, Met-Ala-Trp-Ile-Ser-Leu-Ile-Leu-Ser-Leu-Leu-Ala-Leu-Ser-Ser-Gly-Ala-Ile-Ser (19 residues); in MOPC-41 kappa L-chain precursor, Met-Asp-Met-Arg-Ala-Pro-Ala-Gln-Ile-Phe-Gly-Phe-Leu-Leu-Leu-Leu-Phe-Pro-Gly-Thr-Arg-Cys (22 residues). The extra pieces in the precursors of MOPC-104E (lambda1), MOPC-41 (kappa), and MOPC-321 (kappa) L-chains differ extensively from each other in their amino acid sequence (65-73%). In addition to this sequence heterogeneity, the extra pieces are characterized by a high percentage of hydrophobic residues: 69% in the MOPC-104E lambda1 L-chain precursor (this report), 73-75% in the kappa L-chain precursors [Schechter, I. & Burstein, Y. (1976) Proc, Natl. Acad. Sci. USA 73, 3273-3277]. The marked hydrophobicity of the extra piece suggests that it may favor interaction of the precursor with cell membranes, in a manner similar to the function of the "hydrophobic domain" of membrane-bound proteins. We propose two possible targets for interaction: (i) the endoplasmic membranes, where the NH2-terminal extra piece is cleaved from the precursor to yield mature protein destined for secretion; (ii) the cell surface membrane, where the intact precursor is anchored by virtue of the hydrophobic extra piece to serve as the antigen-recognizing receptor.

Marked hydrophobicity of the NH2-terminal extra piece of immunoglobulin light-chain precursors: possible physiological functions of the extra piece

mRNAs coding for mouse immunoglobulin light chains direct the cell-free synthesis of precursors in which extra peptide segments precede the NH2-termini of the mature proteins. The abundance (18-30%) of leucine residues in the extra piece indicates that it is quite hydrophobic [Schechter and Burstein (1976) Biochem. Biophys, Res. Commun. 68, 489]. Accordingly, we have determined the positions of all hydrophobic residues by sequencing two k-type light (L)-chain precursors that were labeled with: [3H]Ala, [3H]Val, [3H]Leu, [3H]Ile, [3H]Thr, [3H]Pro, [3H]Phe, [3H]Tyr, [3H]Trp, [35S]Met, and [35S]Cys. The partial sequences (and sizes) of the extra pieces obtained are: in MOPC-321 precursor, Met-X-Thr-X-Thr-Leu-Leu-Leu-Trp-Val-Leu-Leu-Leu-Trp-Val-Pro-X-X-Thr-X-(20 residues; X is unknown); in MOPC-41 precursor, Met-X-Met-X-Ala-Pro-Ala-X-Ile-Phe-X-Phe-Leu-Leu-Leu-Leu-Phe-Pro-X-Thr-X-Cys- (22 residues). Despite the fact that these extra pieces differ extensively in sequence (68%), both of them are highly enriched with hydrophobic residues (75% in MOPC-321, 73% in MOPC-41). This marked hydrophobicity suggests that the extra piece favors interaction of the precursor with cell membranes, in a manner similar to the function of the "hydrophobic domain" of membrane-bound proteins (e.g., glycophorin). We propse that the hydrophobic extra piece directs most precursor molecules to the endoplasmic reticulum, where they are cleaved to yield mature L chain destined for scretion; a few precursor molecules escape cleavage and are embedded in the cell surface to serve as the antigen-recognizing receptor. The probability that the Leu-Leu-Leu-Trp-Val sequence occurs by change is 1.6 X 10(-8). Therefore, the data provide evidnece for duplication of a short DNA segment in the structural gene coding for the MOPC-321 precurosr. Duplication with inversion is also indicated from inverted repetition of the Phe-Lue-Leu sequence in the extra piece of the MPOC-41 precursor.

Partial amino-acid sequence of the precursor of an immunoglobulin light chain containing NH2-terminal pyroglutamic acid

Analyses of amino-acid sequences of the total cell-free products programmed by the mRNA of MOPC-104E gamma light (L)-chain show that over 95% of the products have sequences of a distinct protein that correspond to the L-chain precursor. In this precursor an extra piece is coupled to the NH2-terminus of the mature L-chain. Analyses of products labeled with [3H]alanine, [3H]leucine, and [3H]proline demonstrate that the extra piece is composed of at least 18 residues. Analyses of [35S]methione-labeled product indicate that the extra piece may contain an additional NH2-terminal methionine, which is detected in about 10% of the molecules. Partial recovery of the NJ2-terminal methionine (alanine, leucine, and proline are recovered in yields close to theoretical, greater than 95%) suggests that it is the initiator methionine, which is known to be short lived in eukaryotes due to rapid hydrolysis. Thus, the extra piece seems to be 19 residues in length, and it contains one methionine at the NH2-terminus, three alanines at positions 2, 12, and 17, and five leucines at positions 6, 8, 10, 11, and 13. The close gathering of leucine residues, as well as their abundance (26%), suggest that the extra piece would be quite hydrophobic. Hydrophobicity seems to be a general property of the extra piece, since similar clusters of leucine were found in the precursors of 3 KL-chains (Burstein, Y. & Schechter, I. (1976) Biochem. J. 157, 145-151). The NH2-terminus of the mature MOPC-104E gamma L-chain is blocked by pyroglutamic acid. The fact that in the precursor a peptide segment precedes this NH2-terminus establishes that pyroglutamic acid is not the initiator residue for synthesis of the L-chain. Apparently, the pyroglutamic acid is formed by cyclization of glutamic acid or glutamine during cleavage of the extra piece to yield the mature L-chain.

Amino acid-sequence variability at the N-terminal extra piece of mouse immunoglobulin light-chain precursors of the same and different subgroups

The proteins programmed in the wheat-germ cell-free system by the mRNA coding for the MOPC-63 mouse myeloma L (light) chain were labelled with six radioactive amino acids: [35S]methionine, [4,5-3H]leucine, [3,4-3H]proline, [3-3H]serine, [4,5-3H]isoleucine or [2,3-3H]alanine. Amino acid-sequence analyses showed that over 90% of the total cell-free product was one homogeneous protein, which corresponds to the MOPC-63 L-chain precursor. In this precursor an extra piece, 20 amino acid residues in length, precedes the N-terminus of the mature L chain. The extra piece contains one methionine residue at the N-terminus, six leucine residues, which are clustered in two triplets at positions 6, 7, 8 and 11, 12, 13, one proline residue at position 16, and one serine residue at position 18. The closely gathered leucine residues, as well as their abundance (30%), suggest that the extra-piece moiety is hydrophobic. In the precursors, the extra piece is coupled to the variable region of the L chain. Partial sequences of precursors of L chains of the same and different subgroups that were labelled with the above six radioactive amino acids indicate that the extra piece is part of the variable region. Thus the precursors of MOPC-63 and MOPC-321 L chains, which are of the same subgroup, have extra pieces of identical size (20 residues), and so far their partial sequences are also identical (see above). On the other hand, in the precursor of MOPC-41 L chain, which is of a different subgroup, the extra piece is 22 residues in length. Further, the sequence of the MOPC-41 extra piece differs in at least ten positions from sequences of the extra pieces of the precursors of MOPC-63 and MOPC-321 L chains.