  When a GRB emitter stops emission abruptly, the observer would receive
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 sub-sample 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 decays 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.
