Fig. 7.

Reaction profiles illustrating a model for how protein binding accelerates U3-18S duplex formation by unfolding U3 MINI to U3 MINI* (the first step) rather than stimulating hybridization (the second step). In the absence of protein (dashed grey line), the 4 kcal/mol needed to unfold U3 MINI (Fig. 4a) limits the quantities of U3 MINI* and thus reduces the subsequent product duplex to an undetectable level. In contrast, protein binding (black line) unfolds U3 MINI to form a stable U3 MINI* (Fig. 4c), thereby allowing the hybridization step to occur at the intrinsic rate constant for hybridizing short duplexes (~10⁶ M⁻¹s⁻¹) with energetically favorable transitions from U3 MINI to U3 MINI* to duplex. Because hybridization of the MINI-17-18S duplex in the absence of protein (green dotted line) and of the U3-18S duplex in the presence of protein (black line) share a common rate-limiting step, comparison of these reactions is useful. Superimposing the energy of MINI-17 (asterisk) and of the chaperone complex shows the same barrier for the forward reaction (identical k_on (18S) values); however, a 20-fold increase in k_off (18S) by the chaperone complex, results in 1.7 kcal/mol of product destabilization (Table 1). The energy levels thus differ for the protein bound U3-18S duplex and the protein free MINI-17-18S duplex.