  The protein backbone is described as a smooth curved and twisted line in
three-dimensional (3D) space and characterized by its curvature $\kappa(s)$ and
torsion $\tau(s)$ both expressed as a function of arc length s. It is shown
that the function $\tau(s)$ is sufficient to analyze the contributions of all
amino acids to the conformation of the protein backbone. The characteristic
peak and trough patterns of the $\tau(s)$ diagrams can be translated into a
16-letter code, which provides a rapid identification of helices, strands, and
turns, specifies entry and exit points of secondary structural units, and
determines their regularity in terms of distortions, kinks or breaks. Via
computer encoding, 3D protein structure is projected into a 1D string of
conformational letters. The 3D-1D-projection procedure represents an extension
of the Automated Protein Structure Analysis (APSA) method. APSA has been
applied to describe 155 super secondary structures from 94 proteins and to
compare results with Efimov's classification system of super secondary
structure. The applicability of the latter is demonstrated.
