Question

Stimulated B lymphocytes switch from the synthesis of membrane-bound to secreted antibody molecules by increasing the concentration of CstF complexes that cleave and polyadenylate mRNAs. How does this up-regulation of CstF bring about the production of soluble antibodies?

It favors a strong polyadenylation site in the immunoglobulin primary transcript, creating a longer antibody molecule that is secreted.

It activates a weak polyadenylation site in the immunoglobulin primary transcript and prevents splicing, creating a shorter molecule that is secreted.

It favors a strong polyadenylation site in the immunoglobulin primary transcript, creating a shorter antibody molecule that is secreted.

It activates a weak polyadenylation site in the immunoglobulin primary transcript, creating a longer antibody molecule that is secreted.

It favors a strong polyadenylation site in the immunoglobulin primary transcript, aborting translation and creating a shorter antibody molecule that is secreted.

Answer 1

The B-cell antibodies are initially present as membrane-bound receptors. They have a hydrophobic segment that leads to their incorporation into the membrane.

These receptors initially function in maintaining the cell inactive. Once this receptor binds to an appropriate antigen, downstream signaling is issued. This signaling triggers the maturation of the B-cell into an antibody-secreting plasma cell.

This is brought about by several protein complexes. One such protein complex is the CstF complex. This complex binds to an intronic segment of the mRNA transcript. This segment is a weak polyadenylation site. This prevents the recruitment of the spliceosome complex. As a result, the spliceosome complex then splices the primary transcript without removing this intron. And a polyA tail at this intron is synthesized. As a result, the gene product is truncated. Hence the antibody is smaller and since it lacks the hydrophobic segment, it is secreted outside the cell.

Thus the correct option is b.