Freedom and Phosphorylation -- An Interview with Src

By Olga Kuchment

Two faces of Src. Active and inactive Src, as pictured in Young et.al., 2001

For this interview, I had invited the protein v-Src for a drink at our local pub. v-Src is a very dynamic molecule. It was at its most active, its activation loop phosphorylated and madly waving around.

v-Src: You know, I'm the most important molecule in a Rous Sarcoma Virus infected cell. I help the cell evade signals to die, or help it divide uncontrollably and invade healthy tissue.

OK: That is impressive. The damage one type of rogue molecule can do is incredible. I know the DNA that codes for you has some mutations, which is why you always misbehave. But what causes the overall genetic instability in your tumor cells, making them accumulate more and more mutations?

v-Src: That's not very interesting. Let's talk about me.

At this point, we were unexpectedly joined by c-Src. Not noticing that v-Src was there, it came directly toward me. c-Src takes extreme care of its appearance; not a single loop was out of place. Its SH2 and SH3 domains were docked neatly behind its kinase domain.

c-Src: I've been looking for you. I'm outraged. It's an outrage. People have no appreciation for the good work my family and I do. We work toward cell proliferation, differentiation, survival, and when necessary, cell death. All I ever hear is that I'm a target for cancer drug design!

OK: I apprec...

c-Src: The name "Src" is just a throwback to "sarcoma." I have nothing to do with it. For every one of us that gets out of control due to a couple of mutations, there are millions that do their jobs carefully. Just because v-Src was stolen by the Rous Sarcoma Virus and was the first to be studied...

OK: I agree, and I'm a big fan of your work. I've been studying your regulation for several years, as you know.

Pause.

c-Src: Thank you, that means a lot. But I can't stress it more: I am not affiliated with v-Src. At first we were the same, but when the Rous Sarcoma Virus stole a copy of the src gene from a chicken cell, it switched the tail and made several mutations, and as a result v-Src is completely unregulated. It phosphorylates its substrates always, and pays no regard to cell signals from proteins like Csk. Oh... v-Src! You're here too?

v-Src: Are you jealous of the way I live? I'm free, and you're just a tool. You live your life in the service of the cell. And you spend most of it sitting around with your tail in your SH2 domain.

c-Src: Leisure is underrated.

v-Src: Your regulatory domains hold you back! Your SH2 is bound to the phosphotyrosine in your tail, and your SH3 is bound to the SH2-kinase domain linker, your helix αC is swung out into solvent, and stuck that way!

c-Src: That's simply because I'm waiting for a signal from someone like a growth factor receptor tyrosine kinase. Once the regulatory domains are bound by the right ligand, they release, my kinase domain becomes more active, then my activation loop is more likely to become phosphorylated. You wouldn't understand.

v-Src: Give me your activation loop! I'll phosphorylate it, and...

c-Src: Sorry, maybe some other time. My activation loop is tied up right now.

v-Src: Then I'm leaving. But later, you won't have a choice!

OK: c-Src, I've actually been studying how you get from the inactive state to the active. Could I see it?

c-Src: Ah... No, I really can't stay. I've got to catch a vesicle to the cell membrane.

OK: Wait! Let me see! What happens to the SH2 and SH3 domains, how do you move the αC-helix in, and what do you do with the activation loop? Oh, darn.

So it went. Maybe the pub wasn't the best place to get serious answers from tyrosine kinases. I decided to finish my drink and go back to the computer and the bench. That's when the 12 CaMKII's showed up...

References:

1. Nobel lectures by: Peyton Rous, David Baltimore, Howard M. Temin, J. Michael Bishop, Harold E. Varmus

2. Young et. al. Dynamic coupling between the SH2 and SH3 domains in c-Src and Hck underlies their inactivation by c-terminal tyrosine phosphorylation. Cell(2001) Vol. 105 115-126

3. G. Steven Martin. The hunting of the Src. Nat. Rev. Mol. Cell Biol. 2001 Jun;2(6):467-75.

4. Huse and Kuriyan. Conformational plasticity of protein kinases. Cell(2002) Vol. 109 275-282

5. G. Steven Martin. Personal correspondence.