Question

The maximum secretion rates of proteins are one type of limiting constraint on signaling processes since a cell cannot send a signal faster than it can secrete it. These maximum signaling rates can be estimated with maximum rates of transcription and translation. Consequently, proteins with strong promoters can be synthesized at much higher rates. Immunoglobulins, which are important signaling molecules, have very strong promoters. Their maximum secretion rate is on the order of 2000 to 8000 antibody molecules/cell/second, which corresponds to approximately 1 pg/cell/hr.

Required:

a. What would be the maximum signal (in pg/cell/hr) that a cell could send for the following situations:


1. Signal molecule A has a MW = 100,000 daltons and is secreted at a rate of 1000 molecules/cell/second.

2. Signal molecule B has a MW = 50,000 daltons and is secreted at 5000 molecules/cell/second.


b. Which molecule is secreted at a higher rate? By how much?

c. If equivalent fluxes of both molecules were necessary signals for a cellular-fate process, which would be the limiting molecule?

Answer 1

a.

1. The maximum signal that a cell could send for Signal molecule A is (1000 molecules/cell/second) * (3600 seconds/hour) * (100,000 daltons/molecule) * (1.66E-24 g/dalton) = 6.0E-19 g/cell/hr.

2. The maximum signal that a cell could send for Signal molecule B is (5000 molecules/cell/second) * (3600 seconds/hour) * (50,000 daltons/molecule) * (1.66E-24 g/dalton) = 2.4E-18 g/cell/hr.

b. Signal molecule B is secreted at a higher rate by a factor of 4.

c. If equivalent fluxes of both molecules were necessary signals for a cellular-fate process, Signal molecule A would be the limiting molecule since it has a lower maximum signal per cell per hour.