Biology Reference
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cofactors or tubulin-specific chaperones (TBCs) in a reaction that depends on GTP
hydrolysis by the chaperone-bound
-tubulin (
Bhamidipati, Lewis, & Cowan,
2000; Cowan and Lewis, 2001; Lewis, Tian, & Cowan, 1997; Tian et al., 1997
).
The overall pathway whereby tubulin heterodimers are formed
de novo
in higher eu-
karyotes is depicted in
Fig. 11.1
. Quasi-native intermediates (defined as containing a
nativeGTP-binding pocket) generated as a result ofmultiple cycles ofATP-dependent
interactionwith CCT (
Tian, Vainberg, Tap, Lewis, &Cowan, 1995a
) are captured and
stabilized by TBCA or TBCD (in the case of
b
-tubulin) or by TBCB and TBCE (in the
b
case of
-tubulin). There is free exchange of
-tubulin between TBCA and TBCD and
a
b
of
-tubulin between TBCB and TBCE. TBCE/
and TBCD/
interact with each
a
a
b
FIGURE 11.1
The chaperone-dependent tubulin folding and heterodimer assembly pathway. Nascent
a
-tubulin polypeptides are bound by the chaperone protein prefoldin (blue) (
Vainberg
et al., 1998
) and transferred to the cytosolic chaperonin CCT (orange). As a result of multiple
rounds of ATP-dependent interaction with the chaperonin, the tubulin target proteins partition
to a quasi-native state defined by the acquisition of a native GTP-binding pocket. Quasi-native
folding intermediates are captured and stabilized by TBCB (in the case of
- and
b
-tubulin) or TBCA
(in the case of
b
-tubulin). The tubulin target proteins are transferred by free exchange to
TBCD and TBCE, forming TBCD/
a
; these complexes associate to form a
supercomplex. Entry of TBCC into this supercomplex triggers hydrolysis of GTP in the E-site,
destabilizing it and releasing newly formed
and TBCE/
b
a
-tubulin heterodimers. Following free exchange
of E-site guanine nucleotide, these heterodimers are competent for entry into the microtubule
polymer. Note that TBCD and TBCE are each capable of disrupting the heterodimer in the
back-reaction (shown as purple arrows in this figure); in the presence of TBCC and GTP, this
results in a perpetual cycling of tubulin polypeptides through the supercomplex. The activity
of TBCD is modulated via its interaction with the small Ras family GTPase Arl2 (green). Based
on
Tian et al. (1997)
,
Tian, Bhamidipati, Cowan, and Lewis (1999)
,
Vainberg et al. (1998)
,
Tian, Huang, Parvari, Diaz, and Cowan (2006)
,
Bhamidipati et al. (2000)
,
Cowan and Lewis
(2001)
.
/
a
b
