  When a gamma-ray burst (GRB) emitter stops emission abruptly, the observer
receives rapidly fading emission from high latitudes with respect to the line
of sight, known as the ``curvature effect''. Identifying such emission from GRB
prompt-emission lightcurves would constrain the radius of prompt emission from
the central engine and the composition of GRB jets. We perform a dedicated
search of high-latitude emission (HLE) through spectral and temporal analyses
of a sample of single-pulse bursts detected by the Gamma-ray Burst Monitor on
board the {\it Fermi} satellite. We identify HLE from a subsample of bursts and
constrain the emission radius to be $R_{\rm GRB} \sim (10^{15}-10^{16})$ cm
from the central engine. Some bursts have the HLE decay faster than predicted
by a constant Lorentz factor jet, suggesting that the emission region is
undergoing acceleration during prompt emission. This supports the
Poynting-flux-dominated jet composition for these bursts. The conclusion is
consistent with previous results drawn from spectral-lag modeling of prompt
emission and HLE analysis of X-ray flares.
