Proteins gone wild

The Src protein helped teach the world about the molecular basis of cancer. The animal Src protein, c-Src, was first discovered because its mutant, v-Src, was spread among chickens by the tumor-causing Rous Sarcoma Virus. Both proteins are tyrosine kinases, cell signaling enzymes that activate other enzymes by moving a phosphate from ATP to a tyrosine amino acid. Animals require c-Src activity for proper bone development and T-cell development and activation, among other things. c-Src is only active when it receives specific cellular signals, but v-Src is less inhibited and much more active.

The two faces of Src (adapted from Young et. al., 2001.)

My lab mates and I here at the University of California, Berkeley, study the mechanism of Src activation at the molecular level. Unable to reach c-Src for this interview, I invited v-Src, a very dynamic molecule, for a drink at our local pub. The protein got a little tipsy, and it was waving its phosphorylated activation loop like mad.

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

Kuchment: That is impressive! One type of rogue molecule can cause incredible damage. 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 the infected cells, making them accumulate more and more mutations in various cell signaling proteins?

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 in its appearance; not a single loop was out of place. Its SH2 and SH3 domains were docked neatly behind its kinase domain.

c-Src: Kuchment, I've been looking for you. I am outraged. 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!

Kuchment: 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 the first to be studied doesn't mean it's representative of all of us!

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

A pregnant pause.

c-Src: Thank you, that means a lot. But I can't stress it enough: 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 cut off the tail and made several other mutations. As a result, v-Src is completely unregulated. It phosphorylates its substrates always, without paying attention to signals from proteins like Csk. Oh, v-Src! I didn't see you there.

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 phospho-tyrosine in your tail, your SH3 is bound to the SH2-kinase domain linker, your kinase domain is inactive, 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, then my activation loop is more likely to get phosphorylated so that my kinase domain can become more active. 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 I'll get you later!

Kuchment: c-Src, as you know, I've been studying how you move from the inactive state to the active. Could I see it?

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

Kuchment: Wait! Let me see! What happens to the SH2 and SH3 domains, how do you move the alphaC-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 bench.

Olga Kuchment is a junior specialist in John Kuriyan's lab at the University of California, Berkeley. She does research in molecular biology and in her free time enjoys hanging out with family and friends, taking trips to Point Reyes, and conducting imaginary interviews with the proteins she studies.

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, 105:115-126, 2001.
3. GS Martin, "The hunting of the Src," Nat. Rev. Mol. Cell Biol., 6:467-75, 2001.
4. Huse and Kuriyan, "Conformational plasticity of protein kinases," Cell, 109:275-82, 2002.
5. GS Martin, Personal correspondence.
6. Luke Chao, Personal communication.