14-09-24: This is a review I posted directly on PLOS ONE site five years ago. I'm thinking those get lost over there, and also I wanted to try out reviews on this site, so I am copying it over here.

09-06-09: The authors recently characterized NcKin3, which is the first known, naturally dimeric but non-processive and plus-end motor. In this report, they are leveraging this discovery to study chimeric constructs between NcKin (a dimeric, processive Kinesin-1 motor in the same organism) and NcKin3. They make two different chimeric constructs: one with the head of NcKin and the neck of NcKin3, and the other with head of NcKin3 and neck of NcKin. Importantly, the head included the core motor domain AND the neck linker region.

I congratulate the authors on a lot of very nice work that must have been very difficult! The results they report come from an impressive array of difficult assays spanning single-fluorophore position tracking, single-molecule bead motility assays with optical tweezers, gliding assays, and a variety of ensemble biochemical assays.

Study of the two chimeric constructs, in comparison with the NcKin and NcKin3 wildtypes allowed the authors to gain insight into which parts of the kinesin motor are important for conferring processivity onto dimeric constructs. (And also, which parts are important in NcKin3 for inactivating one of the heads.) As far as I know, these are the very first two chimeras created between these two kinesins and thus open the door for many more investigations into how processivity is regulated in the motor domain, neck-linker, and neck regions. The results here indicate that many more chimeric structures and site-directed mutagenesis studies will be necessary and valuable. Of course, that is a lot of work, but the results here open the door for those further studies.

For me, the most fascinating result was point (iii) on page 4. The authors show that the Head3/Neck1 construct seems to get stuck in a "kinetic dead end." As they say, the kinesin-1 neck appears to confer some elements of processivity, but not all. Combined with the missing elements (which kinesin-3 head lacks), the motor is actually a bit more handicapped, as shown by a gradual decrease in gliding velocity as the concentration of motors is increased.

I also had a couple questions about the paper that I noted previously (see prior article comments):

• Statistical significance of processivity measurements.

• Lack of discussion and comparison with previous Ncd/Kinesin-1 chimera results