Question

1. Describe a way in which a membrane bound signal receptor protein could directly regulate gene expression at the level of transcription activation. If there was an intermediate enzyme in this pathway, what effect could that have?

Answer 1

In the process G protein coupled receptor signalling receptor protein regulate different protein which ultimately regulate the gene expression.  

Here we discuss about : muscle relaxation process which includes synthesis of citrulline synthase gene.

In the process of G protein coupled receptor signalling, 7 Trancemembrane domains of receptor are present which is linked with G Alpha GTP protein. Another enzyme which is known as phospholipase C are present as transmembrane protein and activated by G alpha GTP protein. When signal molecule binds to transmembrane receptor of gpcr then G alpha GDP was converted into G alpha GTP and then phospholipase enzyme breaks inositol phosphate into PIP2 and IP3.

Endoplasmic reticulum stores the calcium ions of cell and the membrane of it contains IP3 gated calcium channels. These channels are generally closed and do not allow outflow of Calcium from endoplasmic reticulum. When IP3 molecule binds to this channel they become open and release calcium into cell cytoplasm. These calcium ions binds to calmodulin protein and activate cam kinase enzyme. Cam kinase enzyme Activates the gene which codes for citrulline synthase enzyme. citruline synthase was produced from the gene ( by the process of GPCR signalling) which converts arginine into citrulline and produces nitric oxide gas.

This NO gas move from cell by the process of diffusion and binds to gpcr receptor of another cell where they do muscle relaxation.( As mentioned in image). It produces cyclic GMP from GTP by the activity of membrane bound guanylyl cyclase enzyme. Cyclic GMP molecule activates protein kinase G by bindin to it. Activated protein kinase G do muscle relaxation.

Go through it multiple times for better understanding. thanks