NTP Caps and Possible Phase Changes at Polymer Ends
The principal topic in Chapter 7 was a treatment of aggregation of NTP subunits accompanied by fast NTPase activity at the polymer tips. Actin and microtubules were believed, at one time, to behave in this relatively simple way. As of this writing, it seems clear that, in both cases, NTP subunits actually penetrate (or survive) into the polymer ends by virtue of addition of subsequent subunits: conversion of an added NTP subunit into an NDP subunit is in fact not fast compared to on-off transitions.1 Thus there may be a collection or “cap” of surviving NTP subunits at each polymer end. Though this much seems clear, further details are in the process of being worked out and are by no means generally agreed upon. For this reason, in this chapter we bypass biochemical details (except for illustrative examples in Section 24) and devote the bulk of the chapter to a consideration of two-phase behavior at the polymer ends. The two phases referred to are a polymer end either with or without an NTP cap. This subject can be dealt with without a commitment to a particular detailed biochemical model; we merely assume that some unspecified biochemical mechanism exists that generates two-phase activity (see Section 24 for examples). Actually, so far there is evidence for two-phase activity in microtubules2–4 but not in actin.
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