A Novel Protein Derived from the MUC1 Gene by Alternative Splicing and Frameshifting

Abstract

Genes that have been designated the name "MUC" code for proteins comprising mucin domains. These proteins may be involved in barrier and protective functions. The first such gene to be characterized and sequenced is the MUC1 gene. Here we report a novel small protein derived from the MUC1 gene by alternative splicing that does not contain the hallmark of mucin proteins, the mucin domain. This protein termed MUC1/ZD retains the same N-terminal MUC1 sequences as all of the other known MUC1 protein isoforms. The common N-terminal sequences comprise the signal peptide and a subsequent stretch of 30 amino acids. In contrast, the MUC1/ZD C-terminal 43 amino acids are novel and result from a reading frameshift engendered by a splicing event that forms MUC1/ZD. The expression of MUC1/ZD at the protein level in human tissues is demonstrated by Western blotting, immunohistochemistry, immunoprecipitation, and an ELISA. Utilization was made of affinity-purified MUC1/ZD-specific polyclonal antibodies as well as two different monoclonal antibodies that are monospecific for the MUC1/ZD protein. The MUC1/ZD protein is expressed in tissues as an oligomeric complex composed of monomers linked by disulfide bonds contributed by MUC1/ZD cysteine residues. MUC1/ZD protein expression did not parallel that of the tandem-repeat array-containing MUC1 protein. Results presented here demonstrate for the first time the expression of a novel MUC1 protein isoform MUC1/ZD, which is generated by an alternative splicing event that both deletes the tandem-repeat array and leads to a C-terminal reading frameshift.Genes classified, for the most part, as MUC genes code for proteins that comprise mucin domains rich in proline, threonine, and serine residues (1). The heavily glycosylated mucin proteins derived from the MUC genes can be divided into those that are secreted from the cell (secreted mucins) and mucins that comprise a transmembrane domain that anchors them to the cell membrane (transmembrane mucins). The archetype membrane-tethered mucin, which was the one to be first characterized and sequenced, is derived from the MUC1 gene (2-5). It is a transmembrane protein that contains the serine-threonine-rich tandem-repeat region in its extracellular domain (Fig. 1A, MUC1/REP) and also comprises a 72 amino acid tail that can be tyrosine-phosphorylated (6, 7). The phosphorylated cytoplasmic domain subsequently interacts with second messenger proteins (6 -23), thereby relaying a signal to the nucleus that modifies gene expression.Challenging the classical definition of a mucin gene is the discovery of MUC1 mRNAs, which although transcribed from a mucin gene are devoid of a tandem-repeat array (24 -32). Alternative splicing generates these MUC1-derived mRNAs that no longer code for tandem-repeat array-containing mucin proteins. The prime example is the tandem-repeat array-deleted MUC1/Y isoform, which is expressed both as mRNA and protein (24 -26, 30, 31). The MUC1/Y protein is similar to MUC1/ REP in that it also contains the transmembrane and cytop...

Expression of Vicia villosa agglutinin (VVA)-binding glycoprotein in primary breast cancer cells in relation to lymphatic metastasis: is atypical MUC1 bearing Tn antigen a receptor of VVA?

“…The molecular size of MUC1 varies between 200 and 450 kDa under reducing and denaturing conditions according to number of tandem repeats, as it is well known [30,43,44]. However, we and other researchers have discovered a very small MUC1 [30,[45][46][47][48]. We are interested in small MUC1 forms in relation to the metastatic potential of breast cancer cells in lymph nodes and liver [7,30].…”

“…Second is fragmentation of MUC1 by a proteolytic enzyme, which may yield several small MUC1 fragments [43,44]. Third is splice variants of MUC1: several workers recently discovered MUC/Y, which included small sizes of MUC1s such as $30 and 33 kDa [46][47][48].…”

Alternative splicing: current perspectives

“…As a result of use of an alternative splice acceptor site, the MUC1 gene encodes a truncated mRNA transcript, introducing a frame-shift, that is translated into protein. (102) The FMR1 gene exon 14 is non-symmetrical and when spliced out introduces a frame-shift. Interestingly, the absence of exon 14 changed the localization of the transcript from the cytoplasm to the nucleus.…”

Identification and characterization of a -1 reading frameshift in the heavy chain constant region of an IgG1 recombinant monoclonal antibody produced in CHO cells

“…In addition to nucleotide insertion/deletion, aberrant splicing of the primary transcript can also lead to a frameshift if the normal reading frame was disrupted at the joint site. 18,36 On the translation level, erroneous movements of ribosomes can also lead to frameshifts. 23,24,[37][38][39][40][41] Such frameshifts can be divided into programmed frameshift (PFS) and incidental frameshift.…”