S13
S13 is composed of 3 alpha helices to form one globular domain. Most proteins also include long extensions, and in S13, these take the form of extended carboxy- or amino- terminal helices. These tails are important for contact with RNA. They probably help to stabilize tertiary structure as they can probe into the RNA, approaching many RNA elements. Furthermore, S13 interacts with multiple proteins, including S3, S4, S5, S7, S8, S9, S11, S12, S17, S19, and S21.
(Wimberly, 2000)

S16
Ribosomal protein S16 consists of two alpha helices and five beta strands that are arranged in an antiparallel/parallel sheet. Its nearest protein neighbors are S4 and S20. After these primary binding proteins bind to the 16S rRNA, S16 can take its place in a crevice near helix 21. The binding of S16 causes a conformational change in the ribosome, bringing it one step closer to its final shape.
(Allard, 2000)
(Stern, 1988)

S18
The S18 protein consists of four alpha helices and is surrounded by an ordered polypeptide coil. It becomes involved in the assembly of the growing ribosome after S15 has bound to the rRNA. It cooperatively binds to S6 along its beta sheet and makes contact to the RNA backbone in the upper three helix junction of helices 20,21, and 22. It interactions with S6 are characterized by van der waals forces and salt bridge interactions. It is proposed that S18 can only fold once it is bound to S6 to make a heterodimer. S18 also interacts with S14, S11, and S21.
(Agalarov, 2000)
(Wimberly, 2000)

S19
S19 contains 93 amino acids and is a 10.6 kDa protein. While it does bind to the 3' major domain of the 16s rRNA, it can not bind unless S7 has already done so, creating the S19-binding site. It is located close to S14 and forms a protein pair with S13. S19's secondary structure consists of an alpha helix and three B strands in a parallel/antiparallel pattern. The protein has long disordered tails which are important for interactions with other ribosomal proteins.
(Helgstrand, 1999)