Question

1.) You are studying P120, a protein of 120 KDa that is continuously synthesized in eukaryotic cells. Its cellular concentration, however, is maintained very low because P120 has a very short half-life of 1 minute. You observe that treating cells with Bortezomib, a drug that blocks the 26S proteasome activity, results in a rapid increase of the cytoplasmic concentration of P120. Based on this result, which hypothesis would you propose to explain the control of the half-life of P120?
2.) You then observe that in Bortezomib-treated cells P120 displays a greater molecular mass than in untreated cells. Why?
3.) Indicate at least two general concepts of cell biology that are exemplified by the Ras and NFkB pathways.

Answer 1

Answer:

1. P120 is degraded in the 26S proteasome

2. The 26S proteasome has a major role in protein degradation and is critical for protein homeostasis

3. Cell cycle and DNA replication are cellular processes regulated by the Ras and NFkB pathways

Explanation:

The proliferation-associated nucleolar protein (p120) is a protein known to be expressed during the interphase of the cell cycle, specifically in G1 and early S phase, where any problem with DNA replication trigger a checkpoint, i.e., a molecular cascade of signaling events that suspend DNA replication until the problem is resolved. In mammalian cells, the 26S proteasome is responsible for catalyzing protein degradation of about 80% (or even more) of their proteins. The 26S proteasome acts to degrade rapidly misfolded and regulatory proteins involved in the cell cycle, thereby having a major role in protein homeostasis and in the control of cellular processes. It is for that reason that inhibitors that block 26S proteasome function have shown to be useful as therapeutic agents in diseases associated with the failure of protein degradation mechanisms (e.g., multiple myeloma). The NF-κB are highly conserved transcription factors capable of regulating different cellular processes including, among others, cellular growth, inflammatory responses and apoptosis. Moreover, the MAPK/ERK pathway is able to transduce different signals received on the cell surface to the nucleus. The MAPK/ERK pathway is activated when a singling molecule binds to a cell receptor which triggers a signaling cascade that ends when a transcription factor induces the expression of target genes, ultimately producing a response in the cell (for example, the progression through the cell cycle).