  In the hierarchical theory of galaxy formation, a galaxy overdensity is a
hallmark of a massive cosmic structure. However, it is less well understood how
different types of galaxies trace the underlying large-scale structure.
Motivated by the discovery of a z=3.13 protocluster, we examine how the same
structure is populated by Ly$\alpha$-emitting galaxies (LAEs). To this end, we
have undertaken a deep narrow-band imaging survey sampling Ly$\alpha$ emission
at this redshift. Of the 93 LAE candidates within a 36'x36'~(70x70~Mpc^2)
field, 21 galaxies form a significant surface overdensity (delta_g=3.3+/-0.9),
which is spatially segregated from the Lyman break galaxy (LBG) overdensity.
One possible interpretation is that they trace two separate structures of
comparable masses (~ 10^{15}M_sun) where the latter is hosted by a halo
assembled at an earlier time. We speculate that the dearth of LAEs in the LBG
overdensity region may signal the role of halo assembly bias in galaxy
formation, which would suggest that different search techniques may be biased
accordingly to the formation age or dynamical state of the host halo. The
median Ly$\alpha$- and UV luminosity is 30--70\% higher for the protocluster
LAEs relative to the field. This difference cannot be explained by the galaxy
overdensity alone, and may require a top-heavy mass function, higher star
formation efficiency for protocluster halos, or suppression of galaxy formation
in low-mass halos. A luminous Ly$\alpha$ blob and an ultramassive galaxy found
in this region paint a picture consistent with the expected early growth of
galaxies in clusters.
